Proton Mail & Apple Mail.app Mailbox Issues

I'm subscribed to Proton Mail. Their system has my Mail.app client showing nothing in the Inbox while All mail has unread messages. Not sure how this works, and Proton blames it on the client application.

With Mail.app closed, I head to the Proton web interface, where it currently shows 9 in Inbox, and 9 in All Mail. So Proton seems to be legit, and Mail.app screws this up somehow.

How can I fix this?

Cheers