I had the same issue. You need to downgrade xen-hvm-stubdom-linux and xen-hvm-stubdom-linux-full to the previous version 1.2.3-1.fc32 and then apply patch from this guide:
Contents/windows-gaming-hvm.md at master · Qubes-Community/Contents · GitHub
AppVM with GPU pass-through crashes when more than 3.5 GB (3584MB) of RAM is assigned to it · Issue #4321 · QubesOS/qubes-issues · GitHub
1 Like