Did the behaviour of RigidBody2D combined with PinJoints2D changed between Godot 2 (stable) and 3 (alpha2)?

Question

System

Windows 7 x64
Godot_v3.0-alpha2_win64

I tested this with Godot 2 and it works just fine. Godot 3 (alpha2) gave me some headaches though.

Scene

I use the following scene structure:

Please note:

• World is a Node2D, A and B are RigidBody2D
• A is pinned to World
• B is pinned to A
• The marked RigidBody2D inside A is not wanted!

Script

This is the script attached to World:

extends Node2D

func _physics_process(delta):
    $A.rotate(delta)

Problem

This only works if I add another RigidBody2D to A. If it is not there, A rotates but B stays at its place.
With Godot 2 everything works fine without that additional RigidBody inside A.

Media

Minimal project

Download the project's .zip from Zippyshare.com

Comments

Just a little addition:
I totally missed the point, that a RigidBody2D has no mass when it has no CollisionShape attached. This means I didn't understand the whole concept till now. But I have to say that the described behaviour is still strange...