Change 749775 had a related patch set uploaded (by Func; author: Func):

[mediawiki/extensions/BetaFeatures@master] Use native method to disable checkboxes when \"enable most features\" is selected

https://gerrit.wikimedia.org/r/749775

I noticed that my patch for T298300 can also fix this...

Change 749775 merged by jenkins-bot:

[mediawiki/extensions/BetaFeatures@master] Use native method to disable checkboxes when \"enable most features\" is selected

https://gerrit.wikimedia.org/r/749775

This works now, but the interface is still buggy:

1. I enabled "Automatically enable most beta features" in local preferences:
   
2. I visit global preferences – the settings are now visible, but they're all unavailable:
   
3. I can do a funny dance with the checkboxes to make them available, and configure some beta features globally – but it's not clear whether this actually takes effect, as checkboxes become unavailable again after saving and some of the checks change:
4. When I disabled "Automatically enable most beta features" locally again, it's clear that it actually had the desired effect:
   

However, if you change any global preferences without doing the funny dance in step 3, you'll lose the global beta feature configuration.

The other problems may be resolved by the work on T64815 that @Func started. I'm not sure if we should call this resolved, or not?