Not sure if this really solves the problem, but it is an interesting observation. I have it only on some recurring meetings. When I select the meeting, Outlook shows me the reminder time for the series. When I open the meeting and I select
"Just this one", it shows me no reminder. When I open it as a series, it shows me the reminder time again.
Now, when I set a reminder manually for this meeting and save it, it shows me the reminder set for this meeting, but it sees a difference between the reminder for this meeting and for the series, so the reminder box is grayed out for this session.
The interesting observation. When I go now to future sessions for this series, the difference between "Just this one" and the "series" does not seem to be there anymore. As if doing the above update re-syncs the reminder settings for the series...
Interesting little quirk.... In my team (~15 persons on different platforms), no one else had this issue...