I still had a similar problem in versions:
ios 2.25.3.6
NC 17.0.2
It was a problem with the automatic picture upload, pictures were added to the upload list but the upload never took place, the access.log on the server remained empty (I thought about a size or permission problem). I cleared the cache of the Nextcloud app and uploaded the images manually since the last sync. It wasn’t that much and it offers you to keep the same structure.
I did change the srever URL recently, going from example.com/nextcloud to example.com. The redirects work, not sure if that triggered the problem. Nextcloud then was updated (NC 15 through to NC 17), as well as the NC-ios-app. It wasn’t on all ios apps that use my setup, so it is hard to tell what really triggered it.