Displayport over USB-C stopped working - faulty drivers?
Solution 1:
This is a bug as recorded here: https://bugs.launchpad.net/ubuntu/+source/linux-signed-hwe-5.11/+bug/1939297.
It is still present in kernel 5.11.0-34-generic as I experienced today (on Lenovo T14 Gen1 with ThinkPad Thunderbolt 3 Dock Gen 2 and monitor connected via DisplayPort). If I switch to kernel 5.8.0-63 the problem is gone.
Update (2021-09-28):
The issue is fixed for me with kernel 5.11.0-37-generic #41 that arrived for Ubuntu 20.04.2 today.