I have the same problem. I was creating a solar system. For the central star I placed Barnard's star, then massed it up to about 1.33 suns. (This did not change the luminosity, but I also upped the temp so it looked as it should for its size. So I had a 1.33 solar mass star with a hundredth the suns Luminosity) I saved my system, opened it up and found my star had turned into a monster... however it now had a realistic luminosity.
I believe this bug has been around for a while now. Every time an update brings a new feature (eg. composition, stellar evolution) the saving system has to incorporate it into the save file. I don't think the saving feature is a high priority right now, for that exact reason. That's probably why there's issues to be resolved with the saving system.