How to reproduce:
1. Open attached Unity project "prototype-multi-plane"
2. Press File --> Build and Run
3. Open "GameManager.cs" script with Visual Studio
4. Set some breakpoints in the script
5. In Visual Studio press Debug --> Attach Unity Debugger
6. When project stops on break point, in the Autos window select "this"

Additional steps:
7. In the build press Play and Host
8. Press the green "+" button
9. Press "join" for both players
10. Press green "A" button

Expected result: Values of the variables in a build are the same as playing in the Editor.
Actual result: Values differ when playing in a build and in an Editor.

Reproduced with: 2018.3.0a9, 2018.2.5f1.

Note: Can't reproduce on 2017.1 - 2018.1 versions because of errors.