MoreBloodWine I had been having the same issues. Only once I had Libreelec set to 4K, if I played any content (4K or 1080p) the HDMI input would flicker/blink as if I was changing input or pulling at the cable. It would only do it for me when Kodi tried to do something; e.g. subtitles popped up, up next addon popup, if I pressed play/pause. Sometimes once this happened it would go crazy flicking off and on. I tried so many cables, with and without micro adapter. I had been on Reddit and other forums trying to get help and was being told it was the cables or the adapters, I was certain it was Libreelec, but no one else was experiencing the issue I was. I was ready to fill out a return form for my RPi, when I thought I would just wipe my SD card and try a fresh install (this time I noticed it was v9.1.502 I downloaded, last time I had installed v9.1.501). Everything is now working beautifully, I cant believe I had put up with this for the last few weeks... I hope this helps you also and that you get it sorted!
I noticed the beta 2 thread as well. So I'll check it out... thx.
I also have auto update set to disabled bec u never know what's gonna happen with an uodate.