The WeldJoint is entirely physical, so the center of rotation will be the center of mass of the combined system. So, if both objects are exactly the same mass, the center of rotation will be midway between them. If that is what you observe, then it is intended behavior.Specifically it seems the attached object (B) does not rotate about the owner's (A) origin.
If you wish to make one of the entities totally dominant over the other, then here's a few options:
1) Significantly reduce the mass of the connected entity relative to the owner such that the center of mass of the combined system is roughly the same as the owner. Watch out: if the heavy owner ever has to rely on the light child, instability could result. For example, if the owner is propped up off the ground by the child, you'd probably see some jiggling. This can be mitigated by more solver iterations and shorter time steps, but it's better to avoid large mass ratios to begin with if such situations are possible. If you can turn off collision for the child, that would help.
2) Bind the two entities together into a single compound shape. This requires creating a new entity using the constituent shapes with the compound shape entry weight of some nonzero value for the owner and a value of zero for the child. The two objects will then be one rigid object.
3) Teleport/apply velocities such that the child exactly follows the parent's motion. The child's collisions will be unable to influence the parent's motion due to the lack of a physical connection. Also, it could easily end up inside a wall.