I have a qn65s90dafxza and had this same issue with my Denon receiver. After a lot of testing, I found that Samsung was implementing eArc incorrectly. Whenever the video mode changed (resolution, PC game going fullscreen, Xbox turning on HDR), my receiver would mute itself. The only fix was to disable eArc or disable CEC (which disables audio return function, which should not happen). With 'Anynet+ (HDMI CEC)' on, even when I switch HDR modes on the PC (which is connected directly to the '8k' receiver input and not using eArc at all), it mutes the receiver. When I turn off Anynet+ on the Tv, it works normally.
I found my issues were fixed in the firmware update 1201.8 (release notes below were only mentioned in a German Samsung Support thread).
Update version: 1201.8 October 2024
========================
-UI Operation Specification Improvement Applied
-Pop Up OSD Display Specifications Improved
-Improved Mouse Movements Specifications
-Improved Music Frame Device Connections and peration Specification
-Improved 123 Control UI Behaviour Specifications
-Improved SoundBar Connections Operation Specifications
The only thing I see mentioned in the secret release notes which could possibly relate is the last bullet point 'Improved SoundBar Connections'. "Good job" Samsung. At lease they finally fixed it (7 months after release).
