I always save lyrics as embedded and as a text file. When working with a track that has lyrics, if I right-click in the lyrics panel and select "mark as having no lyrics" MB prompts me to confirm that want to remove exiting and mark as having none. When I click OK, the embedded lyrics are removed, but the text file remains. This has been going on for a long as I can remember. This results in my manually finding the track in a file manager and then manually deleting the lyrics.
Unless this behavior is intentional, and I can't think of a reason why it would be, when lyrics are removed from a track, -all- lyrics for that track should be removed.