Yeah NVIDIA drivers is where it's at. Since I use a 5090, I regularly update them. However the HDMI playback has the problems on 2 different studio driver versions, so I can rule that one out.
MS has confirmed some HDMI related issues with Windows 11 25H2. So perhaps what I am seeing is a spill-over effect from these issues (the confirmed issues revolve around DRM protected content). Guess I have to sit tight and hope they fix it.
Oddly enough: games have far less issues than media players. o__O
Oh well, in the mean time I just use my 2 channel chain and pipe music through the RME.