It's been more than five minutes, I opened the menu again and it's still disabled. I'm on an iPhone 12 Pro Max on iOS 16.4, which is the latest iOS.
> even after switching it off
This might be your issue, because after a restart your iPhone no longer has access to your saved Wi-Fi credentials, but it can still use EAP-SIM to join T-Mobile's Wi-Fi network first. But I see your point that it does not respect the "Auto-join" toggle.