^Q^J doesn't enter \n in replacement field of search-and-replace command in the latest JMACS (JOE version 4.4) of the latest Debian; it clears the replacement field instead. Not a major problem, as I can enter \n directly to get the expected result (i.e.: insertion of a linefeed), but a major surprise after so many years - no: so many decades - of coherent behaviour.
This is now fixed in Mercurial.
Fixed in 4.5