I have an 8K Q800T. My receiver is eARC compatible and connected to the eARC HDMI input 3.
Apps built into the TV correctly output 5.1 surround with atmos, but any devices connected to the TV via HDMI (using audio passthrough) get converted to 2 channels.
With PS4 Pro, LPCM 5.1 output gets received as LPCM 2.0 by my receiver after going through the TV. But if I use the Dolby Bitstream output, then the receiver correctly gets 5.1 Dolby. Nintendo Switch, after going through the TV is LPCM 2.0 as well. Connecting either device directly to my receiver works fine.
Note this sounds like the same issue:
I have Q80T and had this same problem. Everything was being downmixed to stereo using eARC, even the built-in apps like Amazon Prime Video. For a workaround, I switched the TV audio output from "TV Speaker" to "Optical" and it started allowing surround sound content. I'm not using the TV speaker or the optical out, but now the multi channel signal is being sent over eARC. Maybe this will help someone else.
I have the same tv and tried using my ps5s hdmi 2.1 cable as the earc cable and everything seems to work perfectly. Now I just need another 2.1 cable for the ps5 video to the TV.
Curious if your receiver is actually getting LPCM 5.1 or still only getting 2.0. In my case the TV apps output correctly and only passthrough devices get downgraded. If I connect my devices directly to my receiver (same cables) they work fine. When my ps5 is using Dolby I can get surround sound through the tv passthrough, but if I swap it to PCM it gets converted to stereo. That said, maybe I'll try messing with the the wires and do another test.