Are personal Exchange mailboxes having FullAccess permissions on their own mailboxes via their UPN rather than "NT AUTHORITY\SELF" a problem?

One user ran into a problem regarding Outlook tasks that would duplicate on a shared mailbox. As it turn out, the issue was only resolved once I removed the mailbox permission that was given through their UPN, and then added it back with "NT Authority\SELF" with PowerShell.

In an attempt to avoid this kind of issue in the future, I ran an analysis of all the user mailboxes to check if other users had the same configuration. Turns out there is, and I'm trying to ascertain wheter this practice is common if it can be problematic.

I couldn't find much on this, other than the couple of cases where this particular issue was encountered regarding multiplying Outlook Tasks.