To follow up, I can confirm that the issue (which has persisted in the subsequent 2.14.19 and 2.14.20 Appimage releases??) is eliminated as I expected when the outdated libnotify.so.4 in the appimage is removed. I used appimagetool to repackage the Joplin Appimage (2.14.20) without the library in question and Joplin then successfully found the system libnotify.so.4, resulting in a notification. Could someone update the official appimages (either drop the lib or replace with an updated one, better to drop it since (AFAIK) it seems to be needed only by notify-send, which is external to the appimage)?