If you load files with auto-scan enabled, open the context menu of one of the unmatched files in the right pane, wait for the release to finish loading and then use the search for similar tracks action you'll get

Traceback (most recent call last):
File "./picard/ui/mainwindow.py", line 827, in show_more_tracks
obj = self.selected_objects[0]
IndexError: list index out of range