Difficulty settings change on old save when starting a new game

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Yesterday I found out that starting a new game messes up the difficulty settings for other saves. I started a new game with easy settings to test some things, but when I loaded my regular career I was shocked to see that the difficulty changed from 93% to 75%. Creating a new game again, this time with hardcore settings, moved the old save up to 91%. This behaviour is very annoying, as it prevents creating multiple saves with different difficulties.

Edit: I'm not using mods, so they can't be the issue.