Not sure if this will fix the problem for everyone, but I have just figured out this issue of one of my customers. I compared the Outlook Calendar against Teams calendar. In the end, there was a "ghost appointment" in Teams calendar which, for some reason, was not synched with Outlook's. I just deleted the "ghost appointment" from Teams calendar and things got back to work as they should. Sneaky issue.
Not sure if this will fix the problem for everyone, but I have just figured out this issue of one of my customers. I compared the Outlook Calendar against Teams calendar. In the end, there was a "ghost appointment" in Teams calendar which, for some reason, was not synched with Outlook's. I just deleted the "ghost appointment" from Teams calendar and things got back to work as they should. Sneaky issue.