Well, I'm not sure, if this is a bug, but I saved a game, ended the game, loaded the saved game and saved again. Then I compared these sdmp-files and "kompare" (kde diff program) says: there are some differences.
Differences like:
statusResearchable="0" <-> statusResearchable="1"
<preDate day="760758" sec="47483" <-> <preDate day="0" sec="0"
I don't know, if this is relevant, but there is a difference.