I posted a very similar question a few months back and had no replies. As far as I know there is no user documentation for the phone apps (I have looked).
It would appear from testing the IOs app that deleting on the phone does not delete on the server. This is different to the behaviour on a laptop or desktop when deleting a file is replicated on the server. The more I thought about it this is the behaviour a user would normally want on a phone.
One other thought; there are settings on the server for the amount of retained history of deleted files. You might want to check your settings but usually even deleted files can be recovered for a period. I have it set to never permanently delete. That way you can recover earlier versions of modified files.
If you are worried and depending on your settings you should be able to recover any phone photos that get deleted if your experience is different to mine