Generic6DOFJoint is behaving in a weird manner when rotating around the Y axis (it's working properly for the Z axis)

[vyhgatfq]

Godot version:
3.1.1
OS/device including version:
Ubuntu 19.10
Issue description:
I have joined two rigid bodies (two basic capsules) together with a Generic6DOFJoint, disabled the Angular Limit Y, enabled the Angular Motor Y. The capsules started rotating, but after doing so for about 90° they snap back approx. 15° and jump around a bit.
Repeating the same procedure with the Z axis yields two properly rotating capsules without jumping and snapping.

Steps to reproduce:
Create two rigid bodies, join them with a Generic6DOFJoint, disable the Angular Limit Y, enable the Angular Motor Y, set Target Velocity to 1, observe the weirdness.
Minimal reproduction project:
joints.zip

[wombatstampede]

I can acknowledge this for Bullet physics. (Didn't get the 6DOF motor running at all with - old -Godot physics.)
It is also visible in the (otherwise completely unrelated) use case here:
#28444

The motors work properly for x and z axis.

Maybe somehow similar to this one?
#18351

Edit:
Seems to be angular constraint, not motor related. Same applies when I turn off the motor and just add this to the body:

func _physics_process(delta):
	apply_torque_impulse(Vector3(0,30.0*delta,0))

I found this post (from 2012) which seems to indicate that the constraint checks use euler angles and not quaternions. If this is actually still so (which I don't know) then the weirdness would just indicate a moment of "gimbal hell".
https://pybullet.org/Bullet/phpBB3/viewtopic.php?t=8153

For now I would recommend to vyhgatfq to rotate the 6dof joint to be able to just use rotation around the x- or z-axis.

[pwab]

I tested the minimal reproduction project with current Godot 3.2.1.stable and the issue is still reproducible.

[ruanrothmann]

This issue still seems to happen in 4.1 even with Jolt.