OK… I think I’m starting to see what may be the issue here: while I have never used the Source Text character style before, in the couple of computer-related documents this is happening in, I have pasted code from web-sites before, and those have come up in a mono-style font before I manually changed them to my default Ubuntu font via the Font toolbar menu.
Because I wasn’t even really aware of character styles (I did define a few, but never really use them), I just assumed that the only difference was the font. However, I’m guessing that if I pasted some code now, looked at the character styles in the sidebar, I would see it show Source Text rather than No Character Style. While I learned the way to avoid code appearing in another font is to paste as unformatted text, I can’t rule out that Writer still has it as Source Text, even though it was in my default font.
What doesn’t make sense is the absolute randomness of the problem I’ve had - logic dictates that after hitting Enter after one of the offending paragraphs, every new paragraph should then have the character style of Source Text, which isn’t the case. So while that’s weird, I definitely have a greater understanding of how to rectify this, so many thanks for all the help from the people here!
[EDIT: I just tried pasting some code from a forum into a new document, and it does indeed come up as Source Text. though when I paste as unformatted text, it shows No Character Style. Still, I can’t rule out some kind of residue from previous code I pasted normally]