Steps to Reproduce
Bring in two shapes into an empty level. Spread them out. Select one and in Animation mode add a parent constraint and pick the other shape to constrain to. Now nothing in the scene will allow me to set a temp pivot using Alt + MMB click. Even new items I add to the list can’t have their temp pivots moved. Once you delete the items in the scene and bring in a new shape, then it works as normal again.