MacBook Pro M2 Pro using only 1 monitor with Thunderbolt 4 dock

This is a really interesting convoluted problem. My take on what is happening:

We sometimes get into a similar issue with 'stacked' display adapters hung too far away from the computer port.

The COMPUTER ThunderBolt port is capable of being configured into a whole lot of different configurations. It is perfectly capable of forming a DisplayPort over ThunderBolt/USB connection directly, when you plug your adapter in at the Computer.

In this setup, you are now asking a Dock, one step removed from the computer, to create a DisplayPort over ThunderBolt/USB connection. It is not at all clear that this is possible, reliable, or can be done more than once on ONE Computer Thunderbolt port.

[in my opinion] you want to be using a Dock that claims the ability to split the incoming ThunderBolt into Display Interfaces IN THE Dock, and has two or more Direct ports on the Dock for DisplayPort, HDMI or whatever interface you need. Such a Dock WILL likely be able to create up to TWO working interfaces you need, PROVIDED you limit the bandwidth for each display, and use cables that are appropriately short.

The computer needs to create the multiple display interface data stream. By the time you get one level removed, and try to adapt a secondary ThunderBolt port to a display interface, the display data needed are not available, because they needed to be supplied in the computer output.

Anker – Anker PowerExpand 5-in-1 Thunderbolt 4 Mini Dock

One Thunderbolt 4 upstream port, three Thunderbolt 4 downstream ports. "The Thunderbolt 4 downstream port supports media display to a single monitor in up to 8K@30Hz, or to dual monitors in up to 4K@60Hz."

There is a note that it "Doesn’t support M1 MacBooks." I'm guessing that this means that it does not support any M-series Mac that has Thunderbolt 3 ports and is limited to a single external USB-C or Thunderbolt Display. But you've got a MBP with Thunderbolt 4 ports that supports two external displays.

I'm not seeing where the problem would be, right off. Are these the only monitors that you are using with that M2 Pro MacBook Pro? Also, have you checked that the Thunderbolt cable from the MBP to the dock Is plugged into the dock's Thunderbolt upstream port (the one marked with both a lightning bolt symbol and a computer one)?

Then there is this on the Product page in the Q & A:

Q3: I have a laptop with two Thunderbolt 4 ports, but I have 2 external displays that only have HDMI inputs. How do I use them with this dock?

A3: You can use either a USB-C to HDMI cable or a USB-C to HDMI adapter to connect the dock to your external displays.

>>> [no mention of DisplayPort adapters]

Q4: How many external displays can this dock support and what are the maximum resolutions?

A4: This dock supports connection to two external monitors via the USB-C Thunderbolt 4 downstream ports.

Below are the maximum resolutions:

1) Single monitor: Windows - 8K (7680 x 4320 @ 30Hz); macOS - 5K (5120 x 2880 @ 60Hz)

2) Dual monitor: 4K (3840 x 2160 @ 60Hz)

ALSO:

a quick search for the word DisplayPort shows no results on that entire product page

if what you want is to run two displays, neither of which is a ThunderBolt display, the choice of a ThunderBolt-4 dock is the wrong choice. ThunderBolt-4 docks are for multi-sharing a ThunderBolt Bus.

Because of the difference in how Thunderbolt 4 allocates data, every port on a Thunderbolt 4 device that isn’t Thunderbolt must share a single 10GBps slice of USB bandwidth.

from:

https://eshop.macsales.com/blog/87800-whats-the-difference-between-thunderbolt-3-and-thunderbolt-4/

.

Grant Bennet-Alder wrote:

if what you want is to run two displays, neither of which is a ThunderBolt display, the choice of a ThunderBolt-4 dock is the wrong choice. ThunderBolt-4 docks are for multi-sharing a ThunderBolt Bus.

Because of the difference in how Thunderbolt 4 allocates data, every port on a Thunderbolt 4 device that isn’t Thunderbolt must share a single 10GBps slice of USB bandwidth.

from:

https://eshop.macsales.com/blog/87800-whats-the-difference-between-thunderbolt-3-and-thunderbolt-4/

.

That's USB bandwidth. Does the same apply to DisplayPort Alt Mode bandwidth?

Most of the docks that let you plug in two USB-C (DP) displays or adapters seem to be Thunderbolt 4 docks/hubs where the dock has multiple downstream Thunderbolt ports instead of a dedicated DP or HDMI port, and a single Thunderbolt daisy-chaining port. You plug your USB-C (DP) displays or adapters into the downstream TB ports.