I think is normal because Intel Quick Sync can only be used with integrated processor CPU, Intel® 770 in your case.
So when you add an RTX 3080, the iGPU is disabled so Intel Quick Sync too.
Both of them are enabled and i can switch between them in two different monitors.
Also i have the option to select either in Plex settings.
however, what ever i choose, it uses the RTX 3080.