Hi,
I'm using BubbleUPnP version 4.6.5.1 (arm66-v8a).
When choosing to export from "Recently Played" choosing "allow" for the destination folder results in the file never being created.
This outcome is the same regardless of chosen output format and desination folder (internal device / memory card).
Using Enqueue to add to Playlist tab and then exporting works fine.
Hope this is useful to know.