I noticed that I cannot re-create the issue with the latest github code (it keeps working with repeated close/re-launch), I suspect it might be fixed with commit Desktop: Upgrade to Electron 42.3.0 (#15571) · laurent22/joplin@98372a3 · GitHub
public initialize() {
// Workaround: With Electron 42 on Linux, it's necessary to initialize the spellchecker with an empty set of
// languages. Without this, the spellchecker ignores the default provided languages.
this.setLanguages([]);
}
so might start working again in the next pre-release
EDIT: as a workaround, I noticed that just toggling any language on/off from the spellcheck menu also works