@ich777 recommended I open a bug report based on our conversation here
Please see diagnostics attached. If the dev team has time to look at it I'd be very grateful.
Basically, going from 6.9.2 -> 6.10rc2 gives me a code43 on my primary GPU passthrough for any new or previously existing Win10 VM and any new Win11 VM.
I've tried everything I can think of (see thread above) and nothing seems to fix the issue. I've rolled back to 6.9.2 in the meantime, but if you need any further diagnostics I'm happy to upgrade again and try some things out to provide more information.