Looking for help on why my n-channel mosfet is getting destroyed
I'm guessing the diode wasn't soldered down properly on your recent build, or maybe you got some bad parts.  Take one of the boards that failed, replace the FET, and look at the drain with a fast scope while the relay is being turned off.  Then reflow all the solder connections around the diode and maybe even solder wires directly from the diode to the relay and look at the signal again.

You show the schematic, but not the physical layout.  Where is the diode relative to the relay and the FET?  If it too far away, then the inductance to it partially defeats its purpose.

Another possibility is that this was a bad design all along, and now you got some parts where the difference matters.  Try putting a small cap immediately accross the relay.  That will slow down voltage changes so that other parts of the circuit can keep up.  If the relay is off board, then you have to protect the FET drain separately.  This could mean a separate reverse diode on the board an maybe a small cap to ground on the drain.  You don't want to put too much there because it will cause a small surge when switching on, but a few 100 pF to a nF or so should slow down the voltage changes.

What voltage is VBATT?  Why isn't the diode a Schottky?