Steps to reproduce:

1. Open attached project
2. Open scene "youpiTest"
3. In console, notice error: SetLogCallbackDefined is not allowed to be called during serialization, call it from Awake or Start instead. Called from script 'TestAfterSerialize' on game object 'GameObject'

5.3 and lower has this instead: SetLogCallbackDefined can only be called from the main thread.
Constructors and field initializers will be executed from the loading thread when loading a scene.

Names of these callbacks implicate that they're going to be called anywhere but during serialization.

Reproduced with: 5.2.4f1, 5.3.4f1, 5.4.0b10