Simulating mosfet with body bias ltspice
If you set the body voltage of an NMOS transistor to 3V, with the source at ground, then you forward biased the body-source junction. I doubt that SPICE really can model that situation correctly. You probably should not be using more than about 0.5V of voltage from source to body.
On the other hand, how did you calculate the threshold voltage with and without back bias?