I think I know what happened, and it's actually another problem (I think?). What I believe happened was that it was synchronizing the notes in another notebook under a different profile.
To try and get my head around what was happening, here's what I did.
In a brand new instance of Joplin, I created a new profile [call it P1], imported an Evernote notebook and set up a synchronization with Dropbox.
After the synchronization was completed, I created another profile [call it P2], and, again, imported an Evernote notebook and set up a synchronization with Dropbox. I expected the profiles to be self-contained, and I was surprised to see that the notebook I imported in P1 was visible.
So I created a third notebook [call it P3], imported something else, and set up synchronization. Now I can see the notebooks from P1, P2 and P3.
Just to make sure, I set up a P4, and all the notebooks in the previous profiles have appeared, and it was synching everything until I cancelled the synch.
(As far as I can tell, if I don't set up synchronization, then the profiles are fully independent.)
So, is this expected behaviour, seeing notebooks under all the profiles previously created? Is this a Dropbox issue?
How do I get fully independent profiles?
Thanks in advance for any help!