At the moment, the functionality of Proton Photo backup is very basic: it actually only does ... backup :) as in, it sees a photo on your phone CAM folder and it backs it up to Proton Drive into Photos folder - that's it! If you delete the photo in your camera or gallery, it still stays in the Proton photos app on your phone and the web, if you delete photo on the web (or app) - it gets removed from the Proton drive app on your phone (or web) but stays in the CAM folder. Photos can be viewed on the web and in the app, but videos wont play.
I guess, there is nothing to complain as the app does EXACTLY what it says - backup and backup only it does.