We have some serious problem on iOS with Unity. Some users have sporadically changing IDs on their devices. Devices are the same, devices IDs are the same, but from time to time Playfab ID changes by itself.
Our title ID: 4F51
We have a user with first Playfab ID of BA9D249BA885B85C and DeviceID: 503B0EFD-C95B-477F-B507-A6D22BF5BC3C
But then Playfab IDs changed several times to:
2202A1B3F37E946D
716E8D706AA5695
67A1F48B6CAEAA08
648E01AA5CCC9E07
Application was NOT reinstalled.
I assume those IDs are PlayFab ID (master player account ID). May I ask which API your game is using for login? I cannot see the iOS platform user ID on my side, would you check those identities in your Game Manager?
We use LoginWithIOSDeviceID. Then GameCenter login or Apple sign in may be called by a user.
All those IDs have IOS Device ids.
PlayFab won't interfere how Unity retrieves the iOS device ID, as the server simply check the existing of device ID you have defined or get in the request. But I believe GameCetner ID and Apple Sign-in won't keep changing.
If you notice the iOS device ID changes often via the same method, I suggest use AdvertisingIdentifier instead of Systeminfo.deviceUniqueIdentifier. Please also refer to How to fix creating a new user on iOS every time app is uninstalled and installed again - Playfab Community