Thank you Ken !
actor.getBounds() seems to do what I want, you are right, it is in our case in world coordinate,
and is updated as soon as actor.setPosition(x, y, z) is called.
I still don’t get what does actor.getCenter(), I will look into the source code.
Also, I have also to look why, in our code, actor.getPosition() gives something really different than the world positions.
Anyway I think we can proceed and this case is solved, thank you all !