I observe exactly this problem for at least a year now with all versions of Addressables, currently with 1.16.19 and Unity 2021.1.11f1. To be speciffic, the content of the window Addressables Groups completely dissapears from time to time.
You can try this…
I also had suffered from this issue for a log time and almost on a weekly basis, what finally solved it was to completely remove the addressables package from the project (the initial package I installed was one of the first with lots of bugs)
when I reimported the newest version at the time the issue just disappeared and never came back (about a year ago).
My guess is that some of the addressables old (and buggy) config files caused it and once I reset it completely it solved it.
My Addressables groups just disappeared on my after switching platform from iOS to Standalone. I am on the latest verified version of the addressables package, 1.16.19.