I'm having a similar problem with HANDSHAKE between my TCL 617C (2018) and a Pioneer VSX-44 AVR. What's driving me nuts is that the CEC functionality is working. ie. when I adjust volume using the AVR, it displays the changes on the TV screen. I have turned on ARC in the AVR settings. I have selected ARC (HDMI 3: RECEIVER) as my audio output on the TCL. Yet when highlighting that option the TV declares "Your audio device is not connected to the ARC port!" and I get zero sound.
When connecting my PS5 directly to the AVR I get full DD + audio. So it's not the AVR, it's an ARC HANDSHAKE problem between the TV and the AVR.
This is all extra annoying because until this week, I was able to get normal DD surround sound by connecting the TV and AVR via optical cable. I don't know if it's because of Netflix, Prime etc. upgrading to DD+, or a firmware update from Roku, but the TV seems unable to decode DD + to DD, so the TV spits out a 2 channel stereo track instead over the optical cable.
I was hoping to solve this with ARC but now instead I've gone from 2 channel to 0 channel...
If anyone has run into this I would love some advice. Thanks.