Office 365 - User loses permission for shared calendar

Just wanted to see if anyone else had this issue.

We have a user that is using OWA and has a shared calendar that will keep losing permissions. They will see the event, but all the details will be missing.

The quick fix is to remove the calendar and share it back to the user.

The shared calendar was originally shared out from a Shared Mailbox. I had recreated it using a licensed account, but the user will still lose permissions after a few days.

I have also shared the calendar from OWA and from Outlook 2016 and the issue will still come up.

The calendar that is being shared is not the primary calendar. 

This is a cloud-based Office 365 subscription. 

Does anyone have any recommendations for a fix or something I might be missing?