USB-C to DisplayPort connection (16" MBP to Samsung Odyssey G91SD) stopped working after OS Upgrade 15.6.1 -> 15.7.2

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USB-C to DisplayPort connection (16" MBP to Samsung Odyssey G91SD) stopped working after OS Upgrade 15.6.1 -> 15.7.2

Summary: USB-C to DisplayPort connection to my monitor stopped working after OS Upgrade

I have a Samsung Odyssey G91SD 49" 5120 x 1440 monitor that I use with two M4 MacBook Pros. (A 16" M4 Pro from work, and a 14" M4 personal MBP). I have traditionally connected the monitor with a DisplayPort to USB-C cable, it worked fine for several months.

One morning it stopped connecting to my work laptop, but only my work laptop, via DisplayPort. Detailed symptoms below. I am 95% certain that this happened after an overnight upgrade from Sequoia 15.6.1 to 15.7.2 in November. My workaround is using an HDMI cable.

The main consequence/concern:

I now fear updating the OS on my personal laptop, and have been blocking updates for two months. This is a security risk! But I game on my personal laptop, and so keeping the bandwidth on this $1000 monitor is important to me. I fear if I update the OS, I will lose DisplayPort and have to switch to HDMI, which is significantly lower bandwidth. But that's also starting to make me nervous for security reasons.

Symptoms:

No picture on monitor
The monitor's menu shows that a computer is connected to DisplayPort input
Monitor does not appear in Displays preferences on the mac
Monitor does not appear in `system_profiler SPDisplaysDataType`

Only this specific combination (OS >= 15.7.2, this monitor, displayport cable) is affected

Switch to the other laptop running 15.6.1: WORKS
Switch to another (lower res) monitor: WORKS
Switch to HDMI instead of DisplayPort: WORKS

Combinations tried:

SUCCEEDS - Both MBPs, Any OS version, HDMI Cable
SUCCEEDS - Personal 14" MBP, Sequoia 15.6.1, USB-C to DisplayPort Cable
SUCCEEDS - Work 16" MBP, Any OS Version, USB-C to DisplayPort Cable, other monitors I have
FAILS - Work 16" MBP, Tahoe 26.0.1, USB-C to DisplayPort Cable
FAILS - Work 16" MBP, Sequoia 15.7.2, USB-C to DisplayPort Cable
PREVIOUSLY SUCCEEDED - Work 16" MBP, Sequoia 15.6.1, USB-C to DisplayPort CABLE

I only say "Previously succeeded" because I cannot downgrade the work MBP to 15.6.1 to test that configuration again. But it worked fine - this monitor, this cable - for the first three months I had the 16" MBP.

Other things I have tried:

Many restarts / power cycles, of course (no effect)
Resetting the monitor to factory settings (no effect)
Upgrading the monitor's firmware to the latest version (no effect)
Trying all possible monitor's DisplayPort version setting with the laptop (no effect)
Trying all USB-C ports (no effect. All 3 ports on my personal MBP connect, all three ports my work MBP don't connect)
Upgrading to a higher bandwidth cable, now using a 16k-capable DisplayPort 2.1 cable. (no effect)
System Settings -> Displays -> Hold Option -> Detect Displays (no effect)
Many, many combinations of the above actions. Very nearly every combination except for irreversible changes, like OS upgrade

My current conclusion:

This looks to me like there was a regression in how MacOS manages DisplayPort connection or resolution negotiation over USB alt mode, and it happened between 15.6.1 and 15.7.2 and has not been fixed in Tahoe. But it may only affect a small number of monitor models, maybe even only this one model. While the Samsung Odyssey 49" monitors are pretty popular, this specific one is a slight variant that was only sold online at Costco. So perhaps there's a small difference with its firmware. So it's possible Apple is not aware. It's also possible there's something else wrong with my setup that I haven't figured out yet.

I am tempted to make a full backup of my 15.6.1 installation on my personal laptop so I can test the upgrade to 15.7.2 and confirm whether that really is the problem. But I'm afraid if I do, I won't be able to downgrade again successfully.

Any suggestions?

I will put some additional tech notes in comments.

MacBook Pro 14″

Posted on Feb 4, 2026 11:10 AM

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Answer 1

Top-ranking reply

Evan Dorn Author

Level 1

9 points

Feb 4, 2026 12:01 PM in response to Evan Dorn

A FIX IS FOUND

While looking up the commands to get the right information from system profiler, an LLM suggested turning off Adaptive Sync in the monitor's menu. I tried that, and the monitor reconnected to the work MBP right away. So it seems that there likely was a regression in 15.7.2, still unfixed in Tahoe, that prevents the MBP from correctly negotiating with Samsung's Adaptive Sync mode. (The personal laptop in 15.6.1 still works with the monitor whether Adaptive Sync is on or off).

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Answer 2

Feb 4, 2026 12:51 PM in response to Evan Dorn

That display, also known as S49DG91, appears to feature 5120 by 1440 pixels. That is like a "5K" display with the bottom half missing. It can be set to 10 bits/color (HDR=ON) and refresh rate up to 144 Hz are supported.

features include Picture-in-Picture and Picture-By-Picture. But NOT Display Stream Compression (DSC).

video interfaces include

HDMI 2.1

HDMI 2.1 (micro)

DisplayPort 1.4

Both HDMI and DisplayPort (used inside Thunderbolt and USB-C) are packet interfaces. In each case, the interface runs at one of a few Fixed speeds, and if there is LESS data than fully fills every packet at that speed, then packets are (generally) omitted, or stuffed with null data.

You can not run combinations of resolution, bits/color, AND refresh rate that exceed the top speed of the pathway provided. If the display supports Display Stream Compression (DSC), you can get slightly higher data rates (seen as combinations of resolution, bits/color, and refresh rate).

In all cases, use of cables that are sub-standard or longer than allowed can cause the Mac to drop the connection. Comparing to performance under Windows is nonsense, because Windows does not check for errors in transmission.

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Answer 3

Feb 4, 2026 12:53 PM in response to Evan Dorn

if you have an M2 or later Mac. its HDMI port can supply the very fastest display data rate available form a Mac. BUT-- it requires a special cable:

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.

[in my opinion] you should use the available HDMI port and an ULTRA cable for that display.

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Answer 4

Feb 4, 2026 1:09 PM in response to Evan Dorn

When you use a USB-C adapter/cable (as opposed to a ThunderBolt adapter/cable or Dock) you limit the resolutions and refresh rates that are possible to the quantum step HBR2 speed, of about 17.28 G bits/sec. A standard 5K display can only run at 47 Hz in 8 bit color that way.

Your display can achieve 60 Hz at 10 bits/color, but exceeds the data rate by 1 percent or more for 75 Hz.

USB-C data cables or USB-C adapter/cables for this use are limited to ONE meter maximum. any longer, and they may start to make random errors, which will cause the Mac to to Drop the display.

I recommend the HDMI port for this specific display. In M2 and later Macs, has been designed to run slightly faster than a Thunderbolt port can support, and extremely short cables are not a strong a requirement, because it runs at higher voltages.

.

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