I’m having a similar issue. Sync doesn’t actually sync. I believe that in my case, it is an issue with Catalina.
I have created a new note in Joplin on the desktop (running Mac OS 10.15.1). Then I click the Synchronise button.
Completed: 11/11/2019 15:13
Last error: FetchError: request to https://api.dropboxapi.com/2/files/create_folder_v2 failed, reason: self signed certificate in certificate chain
I believe that the “self signed certificate” is the issue.
I also created a note on an iPhone (running 13.2.2). That note seems to have sync correctly (it appears to be in Dropbox). However, it is not showing up on the Mac.
Mac (running 15.10.1)
iPhone (running 13.2.2)
Dropbox for sync
Joplin 1.0.160 (prod, darwin)