Using Classic Outlook on Windows for personal email, calendar, and contact management
Same here, but it's from any non-Microsoft email service. I am not using the Outlook desktop app. I am using Windows calendar and/or the O365 calendar with the Edge browser. No change if I accept the invite from the Outlook mobile app, either.
O365 Exchange just doesn't recognize an accepted calendar invite from an outside organization, at least non-Microsoft senders, and assign a reminder to it. I will report this in the Feedback hub but there is a character limit.