GPU-Accelerated transcoding with 4K works, but not with 1080

[TheDreadPirate]

Thank you for clarifying!

That explains quite a bit; but is this normal that audio transcoding uses 30% of my Ryzen 3750H CPU? Or is it rather the recomposing of the stream? Forcing the GPU to be used wouldn't change to load as the audio still will be transcoded with the CPU hm?

Audio transcoding is a CPU task.  Depending on how fast your system can read and write your CPU will churn through the audio as fast as it can.  So you might have 30% CPU usage for like a minute as it churns through the audio at 50x real time.