I changed back to the folder on SD card, picked the default profile, and restarted the server.

Mapbox Streets (Default) still shows the ambiance background, but switching to OSM Scout Day gives me a white background. And then, after approx 15 seconds, the map appears!

If I then switch back to Mapbox there is a delay of 15 seconds to 2+ minutes before the background turns transparent again. The timing is not consistent and I sometimes get warnings about PM being unresponsive. Sometimes it seems as if the change happens when I go to the home screen, then back to PM.

So definitely something strange going on. I'll add one region at a time to my new folder and see how that goes.