The Nextcloud iOS app does it. For some reason it requires the location permission "all the time" for that, presumably as a way to "wake" the app from time to time?
I decided to try Nextcloud exactly because of this. My problem with it is more that the whole thing is a bit unreliable. Like once in a while the app will get into a state where the only way I found to recover is to just erase everything and re-sync everything. And the app will resend ALL the pictures, even though they are already on the server.
And I can't do that with my family members' phones. It doesn't matter to me if the app takes a month to sync the photos, but it has to require zero maintenance. I can deal with the server side, but I need it to "just work" on the smartphones.
Searching for "nextcloud ios background sync" shows a whole bunch of forum posts and bug reports about it not working well unless you have the application open.
For something that works well it seems like a ton of people have a lot of issues with it. Are you sure you're on the latest iOS version? Seems like people experience the issues when they're on a later version.
I don't know it for sure, but I think it wasn't working until a recent update (like 6 months ago). Also an unintuitive thing is that the location permission has to be given to the app "all the time", and I think I had to manually go into the settings to set it up.
I think that the forum posts may be old, and/or a bunch of them may come from users who did not know that they had to set the location permission this way (which admittedly is unintuitive for photo syncing).
Background support for non-Apple apps is best-effort at best, and explicitly discouraged in the docs. The rate of silent push notifications and other background mechanisms are intentionally not documented and you’re explicitly instructed you not to rely on any current behavior. They make some exceptions to support money makers like Uber and fitness tracking but generally they don’t like you using anything in the background.
Android is more relaxed but the vendors (like Samsung etc) will go around that and implement their own aggressive background killing bots. Sometimes, this causes alarm apps to stop and not wake you up etc.
The main reason is battery life. Tragically, this makes sense due to the cesspool of spam apps that plague their ”curated” app stores. If you’re an app developer who want to use it responsibly you’re in for a world of trouble. I know because I am one of them (well, I consider myself responsible at least).
The Nextcloud app has been running and syncing in the background on 3 iPhones for like 6 months, so they managed to make that work relatively well.
My issue is other bugs that make it painful, including the fact that I cannot trust that Nextcloud will eventually upload the whole photo gallery (it seems like some files regularly get "locked" w.r.t. "webdav", for some reason, and this never resolves).
Do you have to ever open the app though? On iOS/Android?
In my case I would need it to run on the phones of my family members, and they probably will never open the app.