MacBook Air M1 - Netflix - no Dolby 5.1 to Onkyo AVR via HDMI, only stereo

Since a week I have a brand new MacBook Air (M1) running Big Sur 11.2.2. I have connected it as follows to my home cinema setup:

• MacBook Air (Late 2020) - Dell USB-C / HDMI Adapter / HDMI cable
• Onkyo PR-SC5507 AVR
• Panasonic AE3000 projector

In Audio MIDI Setup I can see the Onkyo AVR and I can setup the audio parameters to 6 (5.1) channels. Clicking the test buttons in the setup, I can hear the test tones from the correct speakers. So the setup looks ok.

Now when I play a video from Netflix (Safari), I only get sound from the two front speakers.

So next I tried getting Dolby 5.1 encoded sound. However all the "encoded" options in the Audio MIDI Setup are greyed out. Only the PCM multichannel options are available.

How can I get bitstream (encoded) sound? This article mentions that Dolby 5.1 should be available using a HDMI connection Connect to HDMI from your Mac - Apple Support