I just bought a TCL 55R615 from Best Buy on 10/6/19, and it has been a nightmare dealing with the audio lag/lip sync issues.
My Setup: TCL TV connected directly to a AVR (Denon) via HDMI 3, which has the ARC support. I will also add the AVR is older and doesn't have HMDI 2.0 support.
I tried several different audio setting on TV and my AVR to try and get the audio and video to sync. What I've learned is that the Auto Detect for the TV Audio, as well as many other settings under their do not work. I've finally settled on either Dolby Digital or Dolby Digital/DTS as the only options on the TV that play audio. That being said I still had the audio lag. After hours of troubleshooting many settings I finally changed the HDMI setting under TV Video setting from Auto Detect to 1.4 HDMI. At this point it seems to have fixed the problem for me. My conclusion with the entire ROKU platform is that the auto detect settings they have for both audio and video on that intuitive. So once I forced settings that I know work it seems to be working correctly.
Figured I'd share my experience in case any others are experiencing similar issues.