There I saw a suggestion to downgrade Joplin to version 2.5.
I have so uninstalled the current version of Joplin from my smartphone and installed the 2.5's APK. I've set the syncing via filesync, again, and now it seems to be working.
I have not found more and deep informations regarding this matter, despite I've seen some threads in some places mentioning this is really some kind of bug.
I just would like to confirm, if so, as well as to know if there are some problem in to use this specific version of Joplin (2.5).
Could you please tell us which version you had been using before downgrading to v2.5(the "current" version you mentioned above)? By providing the exact version number you can make the developers find out the problem more easily.