I can reliably reproduce this on a stock Samsung Galaxy S9 running Android 10.
If you force-stop Joplin, then share to Joplin from Firefox, don't edit the note, and use any of Android's back button, Android's back gesture or Joplin's back arrow, the note won't save.
If we repeat the above, but hit the Joplin edit button before returning to Firefox, the note will save.
If Joplin has recently been open, the note saves as expected, but I can't tell how recently it needs to have been used - I guess that's down to Android's application management mechanism.
Hopefully this might help track down the problem? If there's any more details or logs I can supply, please shout!