Mac Mail won't read or create sub-folders

I read mail from an IMAP server: a Synology machine (DSM 6) running Dovecot and using Maildir format. (I was using mbox format on a different server, but recently switched to Maildir on the Synology box.)

Now I am unable to view the contents of subfolders in MacOS Mail.app. If I create a new subfolder on the server directly -- e.g., create a mail directory called .Foo.Bar, filled with emails -- I will see the folder Foo with the mailbox Bar in it in Mac Mail. But, I will not be able to see any of the mail in the mailbox with the app; it appears empty. Nor can I copy mail into that mailbox.

In contrast, I have no trouble seeing the contents of the very same subfolder in the *iOS* Mail app, or with other mail clients. The problem is only in the Mac Mail app.

Similarly, I am not able to create subfolders with the Mac Mail app.

Is this a fundamental limitation of the Mac Mail app -- doesn't play nice with Maildir subfolders?

It would be a drag to flatten out the hierarchy of my mail just for this one mail client!