Tenative Meetings sporadically stop synchronizing to calendar

One of my users synchronizes his Exchange Online account to his iOS devices (an iPhone and an iPad). His tentatively accepted meetings, and meetings he has not yet selected will occasionally stop syncronizing to his iOS calendar. Accepted meetings still synchronize normally after the tentative meets stop synchronizing.

If the Exchange Online account is removed, and then added back in to the iOS device, the tentative meetings will start synchronizing again. After a few days, the tentative meetings will once again stop synchronizing to his calendar, until the account is removed and added back in.

I have found a few discussions from Google searches, but all of them end with removing and then adding the account back on to the device. Does anyone have any insight in to what might be causing the tentative meetings to stop synchronizing to the calendar?