Hi @Bernd_Kaponig,
Thanks for sharing that screenshot, I believe I know what is going on here. As you can see in your original screenshot, you were playing 24-bit content to the Displayport zone. Since the Displayport output only supports 24-bit content as 6ch, this is why it was getting locked into 6ch instead of the expected 2ch:
My follow-up question for you, if you try to output 24-bit content via the TIDAL app instead of Roon and use the Displayport output, do you notice similar behavior in the Audio Midi tool? Note: Please try using MQA content in the TIDAL app with exclusive mode enabled as MQA content is 24-bits but the majority of TIDAL tracks are 16-bits. Exclusive mode can be enabled/disabled by clicking on (More Settings) next to the Displayport output in TIDAL Settings -> Streaming -> Sound Ouptut,