There's no way to replace line breaks with paragraph breaks using the
find&replace function. 

Why would you do it? There are hundreds of reasons - it's just a basic function
of any word processor. (Anyone who wanted to ridicule Open Office could point
this out as an example for shortcomings.)

Some documentation mentions entering "\n" in both the "Search for" and the
"Replace with" field as a way to do it. But that doesn't work (perhaps worked in
former versions?), and is not properly documented as a function of the program
anyhow.

Yes, I did see other users reported this bug before, but then the issue was
marked as "unconfirmed". So this is at least to confirm the existence of the bug.