Fixed joint moving in simulation

Hi!

I have been using fixed joints several times now in order to “glue” rigid bodies onto each other or to a certain point in space (I defined body 1: world, body 2: rigid body). Especially in the latter case I have been noticing, that the rigid body sometimes moves during simulation when colliding with othe moving bodies contradicting to my understanding, that it should be fixed to the point specified in the joint physics.

Is there something I am missing?
I am simply trying to deny any relative movement between the to bodies resp. the body and the certain point in space. Maybe a fixed joint is not the way to go?

Thanks in advance
Kind regards