I use Joplin on 2 machines 1 Win & 1 Linux and I sync them together. When the Linux came out of sync because I did not update the version, I did not notice it before a long time, when I started to wonder why some updates were not copied. When I followed closely the sync, I finally noticed the error message on the bottom left telling me that the sync couldn't be done because the version was too old.
Make this kind of error more obvious for the user. This could be a popup when launching th app if the last sync was not successful and/or a bigger and blinking font for this kind of error.