OK, that didn’t help.
I did discover why multi-line regex ctrl-H \n\n-\n doesn’t work. That converts a pair of LFs into a “\n”, not a LF, an actual backslash “n”.
if I use “\n\n” for the find expression and use a click on the enter key as the Replace Expression, it works just fine.
I would have expected the replace expression to have been interpreted as a regex not literal characters.
Back to the original problem: I have no idea why some files became double spaced, but they were actual double LFs, not a CRLF.
Now that I have mastered ctrl-H all is good.