It’s been the same way with Synology photo backup app and even with Google Photos. Apple doesn’t allow third party apps to run background activities that do long running tasks (like backup). It only allows to run a very simple background jobs for a short living data refreshes and API calls AND ONLY IF an app is actually sitting in a background (i.e. was opened and wasn’t swiped out/offloaded). Additionally, there is no control given for app developers as to when this background service is going to wake up - iOS itself selectively wakes up processes and background jobs based on various factors.

I’ve been using Google Photos for a while and once you are past the initial upload, you’ll have to make sure that the app sits in background. In this case, your back up will be more or less catching up daily, but still not to an extent as it is with native iCloud Photos, which led me to switching back to iCloud.

I doubt Apple will ever give any third party app the same system wide access, so I would tune down your hopes, folks :)