Changes not reflected in iOS

I’m hoping someone can help before i lose my mind!

My app uses firebase, and checks whether a username is available so that they are unique. It’s been working fine for a while, but we moved over to firestore, and now i’ve got an issue.

If i use ionic serve it’s fine, and i can’t use an already claimed username, same if i build the Android version. But it’s just working in iOS, no matter what i do. I’ve removed and re-installed the whole platform, deleted derived data - but on iOS only it’s letting me chose a username that already exists.

Any suggestions gratefully received before i lose the rest of my hair!