Just try to keep track of the fact that you witnessed it “work” for a slight period here (I would suggest refraining though from terming the cessation of operation as “it broke again” since that could be very misleading in this particular case - I feel that I have experienced peculiar behavior with USB attached external webcams that bordered upon “suspicious” that later on stopped happening but while troubleshooting could sometimes lead to a situation where if you utilized precise, skilled timing & excellent observation to suggest courses of testing it eventually could be held for much longer than 2 seconds) & anything like that would tend to prove at the least that the firmware & kernel are not entirely incompatible with this webcam. I also have installed guvcview (another common webcam application) which can sometimes assist about as much or maybe more than cheese in testing & trying to determine what does & does not work.
You may want to tell us about the specific Qubes stats of your installation (you have an installation that is vanilla Qubes 4.1 release candidate 1 with Fedora 34 still set up as the default template & the VM in question is using Fedora 34 as the template upon which it is based & the kernel that is being used is…, y’know exacting precise details which may or may not help much) so that perhaps someone here will have an idea.
Caveat, I am no developer but…I do know that there is a regular issue with webcams but you may simply be out of luck for a while longer. The solution that helps Qubes provide compartmentalization with VMs & works quite well for storage & some lesser USB devices (involving USBIP, it can seem at times almost as if the connection is auto-shutting down after missing key elements of the conversation during setting up - timeout due to lost packets of the header of a protocol conversation or something) doesn’t seem to work quite so well for web cams at the moment. Maybe that will change.
I personally managed to use an external USB webcam throughout the first year of this Covid thing with minor issues but there were times…for the exact same hardware & to some degree the exact same software would not reliably work but for the bulk of a year or two it has performed. I used Zoom via Qubes to chat with my aging mother in a kind of nursing home for 30 minutes at a time or talked with siblings for 40 or so with not much trouble.