I am using ver 5.6.3.1 if that’s relevant. No other details necessary. The question is self-explanatory.
Version 5.6.3.1 isn’t available at the download site of Libre, so that information is probably wrong. Quite often, version information is actually relevant, so try to do your best to be accurate with the little information that you are willing to share. You don’t need to snub the people you expect to answer your question: they are users like you. You probably have autoformat turned on, if you don’t like it, turn it off. You could have found that yourself with very little trouble.
What do you call being restyles/reformatted when I click “enter”?
How do you use Writer: with styles (as an advanced user) or with manual direct formatting (as a beginner)? This is important because the right answer will be different.