Second External Monitor Flashing with M1 Pro MacBook Pro

Another follow-up to document this in case it becomes helpful for anyone in the future.

I discovered some other posts online which seemed to suggest that the S2721Q and similar models had issues with flickering and "blanking" with M1-based Macs. Nobody that I found (as of now 2/2022) seemed to report use with an M1 MacBook Pro, but plenty of users with M1-based Mac minis.

Additionally I found a support article from Dell (https://www.dell.com/support/kbdoc/en-ph/000185689/flickering-or-blanking-video-issue-with-dell-s2721q-qs-monitors-when-connected-to-a-m1-macbook-or-mini-via-usb-c-to-dp-adapter) which suggests solving the problem by switching from Display Port to one of THEIR USB-C to HDMI adapters. I found this to be a silly way for them to try and waste more money from their users rather than actually solving a clear firmware bug.

Ultimately I did try switching to HDMI - USB-C cables. It solved the blanking and introduced a new issue...only one display would operate at 60Hz. Secondary display was forced down to 30Hz. Also looking at system information, the displays were no longer actually "recognized" by the Mac. I returned the HDMI cables.

Ultimately I gave up on the Dells and purchased 2 Samsung S27A70 4K monitors, and 1 day in they appear to be working fine. The Mac system recognizes them over DP, and I've not (yet) experienced any blanking or flickering. They both run at 60Hz, and they work fine with the Thunderbolt dock. Hopefully they keep up without issue.

Now to sell the Dell's. What a pain this has been... I don't know who to be mad at: Apple, Dell, or both of them!

To follow-up on my own post:

Another setting which seems to fix the issue is to set the resolution as "default for the display". The problem here is that what Mac thinks is the "default is 1920 x 1080. Given that this is a 27" 4k monitor, that makes everything on the screen huge, and that's not workable.

Not sure what happened, but on past versions of Mac, it recognized these displays using the Retina UI which seemed to give you the option to scale based on appearance. Now it just lists resolutions and warns you that it may affect performance if you stray from the default. I'm only trying to go up one notch to the 2560 x 1440 setting.

Help!

Another follow-up:

I tried switching the cables between the monitors (just to eliminate another possible cause). Results were exactly the same. Primary monitor is fine, secondary monitor flashes.

Then I tried opening the MacBook so that the external monitors become #2 and #3 with the laptop being primary. This introduced a new issue where both external monitors began flashing. If I understand the specs correctly, the M1 Pro is meant to support 2 additional 4K monitors at 60Hz.

With the laptop closed again I tried inverting the primary and secondary display. Interestingly the "flashing" stayed with the same monitor, so then was the primary display which was flashing.

Mac shows my setup as follows:

• Extended display (1)
• Primary display (2)

Not sure if that's relevant to anything but figured I'd note.