I have just recently bought the pro version of the app and added a google drive of mine to both to my Mac and to my iPhone. I had a movie saved in it and I wanted to see if the syncing the watch history works. I clicked on ‘watched’ as the movie in my phone and then waited for my Mac to reflect upon it and show that I have watched the movie but, it’s still showing the movie as newly added onto the device.
iCloud sync has to finish on both devices. First on the one you marked a video as watched, iCloud sync has to update with the watched status and then on the second device you have to let iCloud sync complete after you launch Infuse so it can gather the updates to watched status from iCloud.
This may take a minute or two but you can check the status on the library settings page.
Infuse used to be able to take advantage of the background app refresh but it broke a while back. You can follow that here.