Outlook Unable to Set Meetings as Private

[PCHealer]

In my organisation the Personal Assistant is able to set some VIP Calendars as private but for other VIP Calendars she can't.  She have full access to all the mailboxes already.

My question is it possible that even if user A have full access permissions over user B's mailbox, their calendar permissions maybe different?   She thinks that previously everyone in the company could view everybody's mailboxes but since GDPR came in about a 2 years ago a change was made to stop this.

Is it possible that a previous Exchange admin has set Calendar permissions separately? and how do I check?  

[#2]

Yes, it's possible to to have a permission issue. Full access does grant you permissions to see private items, but that requires the mailbox to be added as additional account in Outlook (not automapped). And for folder-level/delegate permissions, you need to make sure the corresponding flag is toggled. In other words, use one of the methods outlined here: https://docs.microsoft.com/en-us/exchange/troubleshoot/user-and-shared-mailboxes/private-items-not-d...