I have MacDive on both my Macbook and iPhone (also iPad).
The syncing works on iOS devices, both before and after I upgraded it iOS 13 beta.
However, MacDive on my Mac is not syncing (the progress bar doesn't go all the way to the end).
I unfortunately didn't try sync my Mac recently until I upgraded it to Catalina.
I am not sure if it is a Catalina issue...but iOS 13 and iPadOS 13 work beautifully.
Can you help?
Not syncing to iCloud in macOS Catalina?
Re: Not syncing to iCloud in macOS Catalina?
Ahh...I figured it out...
apparently, macOS Catalina restrict apps to have "Full Disk access" by default. I assume that means access to any files outside the .app container.
I had to manually enable MacDive in "Security & Privacy".
This new security measure actually breaks my Adobe suite of apps too...
Hope this helps other folks.
apparently, macOS Catalina restrict apps to have "Full Disk access" by default. I assume that means access to any files outside the .app container.
I had to manually enable MacDive in "Security & Privacy".
This new security measure actually breaks my Adobe suite of apps too...
Hope this helps other folks.
Re: Not syncing to iCloud in macOS Catalina?
Not sure if this is directly related to the OP's experience or not... but it is related to macOS Catalina and iOS/iPadOS 13 betas. And I know we are talking about unsupported operating systems here so this has been at my own peril. But I hope it helps other Apple heat-seekers out there like me that live over the bleeding edge! 
I sync across 3 devices, Mac, iPhone & iPad. I had been using iCloud for cloud sync. Under the latest betas, iCloud sync was not working. I didn't have the exact issue the OP was discussing where the sync activity bar never made it to the end; mine appeared to fully sync. However, the iPhone and iPad would not pick up the new data; they appeared to complete the sync process but they got no new data. The first solution (hack) I came up with was to do a "Reset Cloud Data" in MacDive on my Mac, then delete & reinstall MacDive on my iPhone & iPad. It would then pull down all the correct data. But the next time I added a new dive on my Mac, I had to do this all over again.
Then I had an idea... what if I tried Dropbox as the location for the cloud sync instead of iCloud? So I reset all 3 of my MacDive clients to use Dropbox and everything seems to be working correctly now; new data downloaded into my Mac syncs to my iPhone & iPad.
Again, this is a work-around for unsupported OS's... but it seems to be working.
I sync across 3 devices, Mac, iPhone & iPad. I had been using iCloud for cloud sync. Under the latest betas, iCloud sync was not working. I didn't have the exact issue the OP was discussing where the sync activity bar never made it to the end; mine appeared to fully sync. However, the iPhone and iPad would not pick up the new data; they appeared to complete the sync process but they got no new data. The first solution (hack) I came up with was to do a "Reset Cloud Data" in MacDive on my Mac, then delete & reinstall MacDive on my iPhone & iPad. It would then pull down all the correct data. But the next time I added a new dive on my Mac, I had to do this all over again.
Then I had an idea... what if I tried Dropbox as the location for the cloud sync instead of iCloud? So I reset all 3 of my MacDive clients to use Dropbox and everything seems to be working correctly now; new data downloaded into my Mac syncs to my iPhone & iPad.
Again, this is a work-around for unsupported OS's... but it seems to be working.
Re: Not syncing to iCloud in macOS Catalina?
iCloud sync works for me on Catalina and iOS 13. Perhaps there’s something special about your configuration or mine.