/// <summary>
/// Enforce "include what you use" rules when PCHUsage is set to ExplicitOrSharedPCH; warns when monolithic headers (Engine.h, UnrealEd.h, etc...)
/// are used, and checks that source files include their matching header first.
/// </summary>
public bool bEnforceIWYU = true;
In ModuleRules.cs
It is active for engine plugins and not active for projects by default. It turns out that the flag should be inverted