Keep gravity diraction after group rotation

Hello all.
This question is maybe more related to AmmoJs, therefore I also posted this on the repository.

I have a group containing items (spheres). I am then able to rotate the group and therefore changing the positions of the items with it.
When I active gravity with vector(0, -100, 0) after the group rotation, the items will fall to the direction the group was rotated to and not to the gravity direction.

Gravity without rotating the group:

Gravity after rotating the group (group base is now facing the camera):

Is this related to .up direction? Both group and items .up vector is always pointing (0, 1, 0).
Any ideas?

Thank you for your help and time!