To reproduce:
1. Open attached project
2. Enable Services > Ads
If getting: Assets/UnityAdsExample.cs(15,31): error CS0246: The type or namespace name `ShowResult' could not be found. Are you missing an assembly reference? error > Reimport All
3. Switch to the iOS platform
4. Navigate to ProjectSettings folder and open UnityConnectSettings.asset file with any text editor
5. Find:
m_GameIds:
AndroidPlayer: 2762329
iPhonePlayer: 2762327
m_GameId: 2762329

These numbers may differ, but the main point is that m_GameId should be the same as AndroidPlayer and different than iPhonePlayer

6. Build for iOS
7. Check the file again and see that the m_GameId is not changed to iPhonePlayer ID
8. Build for iOS again and the m_GameId will change to iPhonePlayer ID

Expected result: m_GameId is changed to the building platform the first time you build it
Actual result: m_GameId only changes to the corresponding platform when building the project for the second time

Reproduced on:
2017.2.3p3 2017.3.2f1 2017.4.10f1 2018.1.9f2 2018.2.5f1 2018.3.0a10 (ID doesn't change to iPhonePlayer after building a couple of times)

MacOS 10.13.5
This also reproduces on windows 10 machine when building for Android. Change m_GameId to iPhonePlayer ID and it will not change when you build the project for the first time to AndroidPlayer. The second time you build it m_GameId will change to AndroidPlayer.