Hey, running a similar setup (CoreELEC → Q990B → LG CX) and I’ve been noticing the same thing regarding the audio sync on CE 21.1.1 stable. It’s incredibly noticeable with the audio tests from these test pattern videos. They are perfectly in sync when played though the TVs internal player (eARC) and when using my X800M2 (passthrough).
Setting a delay after refresh change (Settings > System > Display > Delay after change of refresh rate) to 2 seconds mitigates the issue but it is still slightly out of sync. As I am new to all of this, I was wondering if there were any new workarounds/solutions since you discovered the issue without blacklisting the other refresh rates.
Thank you.