Text Styles de-attach themselves

I have a row of text styles which I apply to all the text I’m using in my designs. Unfortunately, it happened now repeatedly that all at once all text in my file is not linked to my text styles anymore. The text properties are still the same but I have a huge problem if I want to change my text styles now.

I already had to restore previous versions of my file twice now and I have absolutely no clue what causes this issue. Since this is stealing a lot of time and nerve, I would be happy if anyone knows how to solve this issue. Thanks!