We are using the GameSparks for Unity SDK V126.96.36.199 with Unity 2017.3.0f3. We have found that GameSparks seems to interfere with Facebook logins made through the Facebook SDK on iOS 11.3. If GameSparks is in preview mode, the Facebook SDK does not remember us between sessions and we have to log back in to Facebook each time we run the game. When GameSparks is not in preview mode, the Facebook SDK does remember us between sessions. This behavior is very consistent. I'm not sure how GameSparks is interfering with the Facebook SDK, but it certainly is.
To be clear, this issue is not directly related to GameSparks authentication. I'm talking about logging into Facebook using the Facebook SDK for Unity.