Did you set the vendor ID under your CPU flags in your
/etc/pve/qemu-server/*.conf file?
cpu: [cputype=]<enum> [,flags=<+FLAG[;-FLAG...]>] [,hidden=<1|0>] [,hv-vendor-id=<vendor-id>]
cputype=<486 | Broadwell | Broadwell-IBRS | Broadwell-noTSX | Broadwell-noTSX-IBRS | Conroe | EPYC | EPYC-IBPB...