That
That's what I am currently doing I think. The Roku HDMI is going into one of the tv's HDMI inputs,I have the AVR's HDMI out plugged into the tv's HDMI ARC input.
That's not what I'm saying. Plug the roku HDMI directly into your AVR (the media-in HDMI) and see if you get DD+ out of the AVR. This is just for troubleshooting. Do you get DD+ in this configuration?
There's a lot of handshaking (protocols) going on. I'm suspecting the roku isn't getting the right handshake from the AVR (through the TVs ARC) in the configuration you're using. It thinks it's plugged into a TV which is outputting the sound and only sending it stereo. If the roku (or any device) doesn't get the proper handshake it assumes the source is two channels.
Another couple of things to try:
1. If the PS5 working correctly. Try unplugging it from the TV, and plugging the roku into that exact HDMI. Sometimes on TV's only specific HDMIs support specific features. Perhaps the one you're using doesn't support a bitstream passthrough (DD+) to the AVR via the ARC.
2. Check that your roku is set to "passthrough." This is somewhere in the Audio settings on the roku.
3. Check that CEC is enabled on both the roku and the TV. On the roku it's called something like control other devices. Not sure what your TV calls it. This is less likely.