Using Classic Outlook on Windows for personal email, calendar, and contact management
I'm having the same problem with the desktop Outlook. I noticed that the reminder for an individual meeting in a series is getting set to "none" while the series reminder is still at 15. Also the icon indicating a recurring meeting does not have the slash through it to indicate the single instance has been changed.
I've started checking my meetings (single occurrence of a series) every morning and I have found that if any have been set to none, I can close outlook desktop, change the series reminder time in outlook online, then open outlook desktop and the reminders are now set to the new time. This only work temporarily.
I tried /cleanreminders and renaming the profile file to force it to rebuild it but that didn't solve the reminders issue.
I have skype for business running constantly and with COVID all my meetings have been updated with a skype link. I'm starting to wonder if Skype for business isn't causing the issue.