I believe that video output does not itself occur via USB i.e. video altmode repurposes some of the USB bandwidth to carry something approximating DisplayPort.
Dare I ask whether you have captured output from lsusb when it did work?
Is a second display showing up in Settings?
Anything from journalctl -b 0 | grep something-appropriate
(and again whether you have captured journal from when it did work)?
Do you have another computer that is capable of video output over USB-C e.g. a Librem 14?