The mosfets are configured as simple source followers. The mosfet sets the source voltage and output current is conditional on the load.
They are not current-controlled devices (ccs's) and should not be thought of as such. Ever.
How could Vgs possibly be 46V when you said you put ~48V across the heater? Putting ~48V across the heater requires that the source be at 48V and the gate cant possibly be higher than the 48V rail...
Indeed, the Vgs curves only go to like 10V for the IRF510, and that sucking down 10A! What do the Vgs curves look like with your mosfet?
Vgs was still what it normally is, give or take a bit. Vg and Vs were no where near what they normally are which kind of screwed you up a lot. Lots of voltage, then the current came because of the voltage across a resistance... then poof.
I never said the MOSFET were current controlled. I said the source current was dependent on the gate voltage when in saturation mode, which I believe is the case here.
I did not say I put 48v across the heaters. I said the MOSFET went fully ON, so I guessed it was like hooking the heaters directly to the power rails. I forgot about the drop needed for Vgs, and I don't know how much it was. But you're right the heater surely did not see as much voltage.