I accept meetings and days later find them sitting as tentative in my calendar again, which has now caused two double bookings. Sometimes the organizer even has my acceptance on record.
Office 365 with Outlook on desktop and phone. Where do acceptances go to die?
The giveaway is the organizer holding your acceptance while your own calendar disagrees: the response went out fine, then something on your side rewrote the tristate afterwards. Three rewriters cover nearly all cases:
The phone app double handling. When the desktop and a phone mail app both process the same invitation, the slower one can stamp the item back to its own state. Several non Microsoft phone clients treat any calendar item they sync as tentative until touched. Test by accepting a meeting purely from desktop Outlook and leaving the phone untouched for a day: if it holds, the phone app is your rewriter. Moving the phone to the genuine Outlook mobile app resolves it since it processes invitations properly.
A delegate or shared mailbox arrangement. If an assistant or a room booking setup has rights over your calendar, their client's processing can override yours. Check File, Account Settings, Delegate Access for delegates you forgot existed.
Corrupted meeting items in the cache. If only specific recurring series misbehave, the local copies are damaged: remove the series from your calendar, have the organizer resend and accept fresh. And when the pattern is broad and neither test above convicts, your admin can inspect the mailbox's calendar processing server side, Get-CalendarProcessing shows automation stamping states on arrival.
Desktop only test held for three days, so it was the built in mail app on my Android re-stamping everything. Switched to the Outlook app and a week of acceptances have all stuck. Double bookings over.