I have a user A who sent an tentative meeting invitation to User B. User B claimed that she never ever click on declined accepting meeting, however User A experience more than 1 times that User B declined his meeting . User B had shared her calendar with her secretary.
What I did was:
1. No Outlook rule settings on User B and her secretary
2. No auto decline meeting request was set on User B Outlook.
3. Asked User A sent a test meeting to User B with different subject compare to above meeting, no issue.
4. Check at message tracking, there was declined email from User B to User A
Now, I'm out of clue.