I had exactly the same issue previously. I kept losing one of the 5GHz channels in the GUI (both from setup page or any page where should’ve been selectable.) However Wi-Fi Analyzer app spectrum analyzer confirmed it was still running fine, and clients were still connection and working. So I was able to confirm it was actually still running, but just not showing up in the GUI. R8000 reboot would fix for a few days, and then it would “go away” again in the GUI. One of the later test builds seems to have fixed the issue. I’m running the latest test build now, and do not see this issue.
If you run a search, you can probably find the other thread where I reported it...