Hi Mallen32,
Can you please let us know how did you connect Mac Mail to the Office 365 mailbox, via Exchange mode or POP/IMAP mode?
I have tried performing some tests on my Mac Mail client using the Office 365 test account. After I connect the account in Mac Mail using Exchange mode, all the messages/folders can be downloaded without issues.
If you’re using Exchange mode and the issue persists, I suggest you try removing the account from Mac Mail first, then re-connect it to check if there are any improvements.
Here is an article for your reference: Set up email in Mac OS X Mail.