Odd. The only time I've seen an issue similar to this was when a programmable Microsoft or Logitech (I forget which) mouse had the middle mouse button assigned to something else. In programs other than Inventor, it appeared to behave normally, because in many contexts the alternate setting acted like a middle-click. But it didn't work correctly with Inventor in particular.
That's why I was asking about programmable mice. If yours is a basic mouse and otherwise seems to be working, I'm not sure what the issue could be.
Maybe you could try swapping with one of the other identical mice, to see if the behavior sticks with the computer or the mouse.
