Power supply noise caused by external heat pad
Disconnect the heater. If the glitch persists, try a capacitor across the coil driver. If long distance, add a line filter to the coil wires with a common mode choke and twisted pair.
Connect the heater. If a disconnect fixes the problem and if the glitch persists, add a snubber without 120 ohm on AC. Or an LC filter using X rated capacitors.
Use low-ESR capacitors near the relay power draw.