If you're talking about the behavior on https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-betafeatures, I think this is expected – when "Automatically enable most beta features" is checked, you can't uncheck the individual beta features, so their checkboxes become greyed out to indicate this.

Or do you mean that the editor itself would not open when you tried to edit an article? (Note that visual editing is not available in some namespaces, e.g. on talk pages.)

"Visual editing" was only available to me for article https://en.wikipedia.org/wiki/Viterbi_algorithm after I unchecked "Automatically enable most beta features" in my Wikipedia - Preferences, Beta features tab.

But I just checked this again and that is definitely no longer the case: the problem is gone somehow. Thank you.