Sounds like it may not be.
IF I remember correctly, a 3650 shifter will physically bolt up to a T-45 tranny but not work anywhere near right. I think I remember reading that on here, but if I am wrong someone PLEASE correct me.
Another thing... there should be a plastic cup that is either attached to the base of your old shifter when it comes out, or still attached to the transmission where your old shifter sat. Make sure it is still in place, wherever it is and that it did not get lost or moved out of place.
Don't know what else to tell you because there isn't really anything to physically stop the shifter from moving especially if it works fine when before it is installed.