Really odd. This background processing in spite of hitting the abort button seems to happen on v36.1 beta only (for me), but I cannot see what change I could possibly have done. I haven't been poking with the core in v36.1. The problem also appears if you run jAlbum on Java 14, so it isn't that we've moved to Java 23.
Anyway, I've adjusted the code to abort better. Now it seems to behave. Test by replacing your lib/jalbum-core.jar with this one:
https://jalbum.net/download/beta/jalbum-core.jar