Based on my audit for a client I found that a user mailbox was at sometime converted to a resource mailbox. There is no convert button/ link to switch it back. I still don’t know how, or why this would have happened.
Anyways, for someone who may come across this weird issue, here is the fix for it.
Set-Mailbox firstname.lastname@example.org -Type Regular
This will convert it from a shared mailbox, or a resource mailbox to a user mailbox.