I got the answer from support, but will post it here in case anyone else needs it. Since Android 11, top-level folders can't be scanned, only level 2 and below. So you need to create a new second-level folder and set up the folders on your SD card like this:
<my existing top level>
... <new second level>
... ...<music>
And then set the app to scan <new second level>.
<my existing top level>
... <new second level>
... ...<music>
And then set the app to scan <new second level>.