[3.x] [Bullet] Changing 3D gravity in BulletPhysics does not work as expected

[gyrosp]

Godot version: 3.2 RC 2

OS/device including version: Windows 10 / PC

Issue description: I'm trying to make a game where the gravity changes during runtime. This does not work as expected.

• Changing the gravity in _Ready() works fine
• Changing the gravity in _Process(float delta) just slows the default gravity vector
• Changing the gravity in _PhysicsProcess(float delta) freezes the gravity:

    public override void _Ready()
    {
		// Works as expected
        Vector3 lNewGravityVec3 = new Vector3(0,1,0);
        PhysicsServer.AreaSetParam(GetWorld().Space, PhysicsServer.AreaParameter.GravityVector, lNewGravityVec3);
    }

    public override void _Process(float delta)
    {
		// Just 'slows' the gravity
        Vector3 lNewGravityVec3 = new Vector3(0,1,0);
        //PhysicsServer.AreaSetParam(GetWorld().Space, PhysicsServer.AreaParameter.GravityVector, lNewGravityVec3);
    }

    public override void _PhysicsProcess(float delta) {
        // Freezes the gravity
		Vector3 lNewGravityVec3 = new Vector3(0,1,0);
        //PhysicsServer.AreaSetParam(GetWorld().Space, PhysicsServer.AreaParameter.GravityVector, lNewGravityVec3);
	}

Minimal reproduction project:
Physics Test.zip

[Xrayez]

Seems to be mono-specific, can't reproduce with GDScript.

[Zylann]

I can reproduce this too in 3.2.1, following this QA question: https://godotengine.org/qa/66952/how-to-set-3d-physics-gravity-programatically

This only happens with Bullet. GodotPhysics works.