Both of these monitors could be going past the maximum allowed bandwidth, but I think I recall seeing some cases of dual 4k120hz working before. Here's a bit of testing and troubleshooting we can do:

The most common reason a monitor cannot reach full specification is due to a problem with the cables. This can be both the Thunderbolt cable connecting the dock to computer or the monitor cable attaching dock to monitor.

Since you have two identical monitors, let's start with tests there. First, is this behavior happening on just one monitor or is it jumping between both of them? if it's just on the one, what happens if you swap the monitor connections on the dock? Does the behavior follow the monitor or stay with the port?

If the behavior follows the monitor, try swapping the cabling at the monitor itself and see if the behavior stays with the monitor or changes with the cable. If it changes again, that suggests the cause is the cable, so I'd suggest swapping it out for a different cable.

Otherwise, try swapping out the Thunderbolt cable that connects the dock and computer. Sometimes the Thunderbolt connection can have slowdowns caused by the cable itself, which could lead to a behavior like this.

If you don't have another Thunderbolt cable on hand to test with, you can first try swapping the orientation of the Thunderbolt cable. Since Thunderbolt ports are reversible, try flipping the port over on either or both ends of the cable - changing how the port and cable mate can sometimes clear up this behavior.

Beyond that, I recommend that you get in touch with our support team for further diagnosis and troubleshooting. You can best get in touch via email at [email protected]

I think it's the TS4 Dock. Like I said, both monitors are using a HDMI 2.1 cable connected to a HDMI 2.1 to USB-C Adapter.

When I plug both adapters at once to the Mac M1 Pro directly, they both work at 4k@120hz (even 144hz).

When I plug both into the Dock instead, only one works at 144hz. The other goes to 60hz. Tried to set the one that goes to 144hz back to 120hz to see if the other would work at 120hz like that, nothing.

If I unplug the one that is managing the 144hz/120hz, the other immediately displays a 120hz/144hz option.

So it's for sure a bandwidth issue with the dock itself. I'm using a Caldigit branded Thunderbolt cable, this one to be specific:

https://www.amazon.co.uk/Intel-Certified-CalDigit-Thunderbolt-Cable/dp/B08NFQ32PR