I have installed Mobius Sync on 2 iPhones with the intent of synchronizing the databases of KeePassium on the two phones. The sync seems to work fine, but even though I have “Background App refresh” turned on, as soon as I switch to another app, Syncthing on my other devices shows “Device Disconnected”. Does code in the app need to be changed to allow Mobius to truly run in the background even when another app is being used?
AFAIK Apple will not allow apps to run in the background indefinitely. You should check the FAQ on the Mobius site. It is a closed source 3rd party app that wraps around Syncthing so this isn’t really the place to look for Mobius specific support.