Lesley,
I've had what you describe occasionally happen, too. It is a bug. For me, it usually happens with the lyrics that I change affecting lyrics at the beginning of the melody. I agree, it's frustrating.
What I do is when I get lyrics partly entered and correct, I ALWAYS save the song with a new name. Then, I enter some more and save with another new name (something like songname 1.mgu, songname 2.mgu, etc.). While this gives me many files, usually around a dozen, it also means that I can open a previous correct file and begin re-entering from there rather than enter the lyrics from scratch if something goes awry. Also, BIAB files are really tiny by today's standards and don't impact noticeably on disk space.
The issue has been reported to Development. Because it's seemingly random, though, it's very hard to track down and thus solve. I've been working on trying find how to reproduce it so that Team PG can get a handle on it and fix it. So far, reproducibility eludes me. If you stumble across what causes it, please send an email to support@pgmusic.com and let them know.
Regards,
Noel