Apple Mail behaviour: can not create sub-folders

[Joseph Tam]

One of my Apple Mail users recently complained his mail reader
couldn't create sub-folders -- he could only create top-level folders.
Playing around with this, I discovered that I could create folders (
as opposed to mialboxes) *if* I specified mailbox name with a trailing slash.

Has anyone come across this?  Is this related to

 	https://doc.dovecot.org/configuration_manual/mail_location/mbox/mboxchildfolders/

?

Joseph Tam <jtam.home at gmail.com>