Trying to change 1 paragraph style changes entire document

I open a .txt document created in Google Keep (or others). I want to change 1 paragraph to a different style. The entire document changes. Clear Direct Formatting doesn’t help. Same thing happens if I copy/paste into my default template. The entire pasted section changes instead of just the 1 paragraph. When I do a CTRL Z undo the entire pasted section is highlighted.

Libreoffice Version:
windows 10

It seems that instead of paragraph breaks (Enter) you got line breaks (Shift Enter). You can easily check if that’s the case by activating the non printing characters: line breaks show as a “hook” instead of a pilcrow sign.

To fix that, open the full Find&Replace tool (Ctrl H), write \n on both, the “Find” and the “Replace” boxes, under Other Options check Regular Expressions and finally click on Replace All.

This worked, Thank you