LO treats a bunch of dashes, equal signs, or underscores at the start of a line as a request to place a bar in the document. They're hard to get rid of once they appear until you figure out what it's doing.
I just created one using a few dashes. Then, I pressed Ctrl-Z (Undo) and the dashes came back as I had typed them, but the bar can sometimes come back again.
This drove me crazy the first time I ran into it too!
The thing is a bottom of paragraph border. (It's not a bug, It's a feature!)
There are several ways to avoid/remove this. Pick one.
1) The simplest way is to type 1 or more blanks (or anything except a dash) at the start of the line - then the border will never get added.
2) Turn off AutoCorrect which is just trying to be helpful

by adding the border for you.
3) Move your text cursor to the paragraph (just above the line) then
Format->Paragraph->Borders
and then click on the first box - which is no borders.
(You can also select a bunch of paragraphs at once and do this once for all of them.)
See:
https://help.libreoffice.org/Common/..._Lines_in_Text