Filtering out TRIAC's triggering current spike on the mains
You need an inductor in series with the circuit.
To make the mains voltage collapse there is a lot of current flowing into your heater for a short time. An inductor will choke this, while allowing the 60 Hz current to flow.

An example value of 1 mH will present a 0.4 ohm reactance to the mains frequency current, so will drop a few volts with a 2 kW load. But it will present a 1000 ohm reactance to the spike in question, in the &lt;10us time range.
This might be a bit much inductance, requiring a large core and coil, but you could find something suitable in the range 100 to 1000 uH.

Edit: looking at your circuit, you could place the inductor on the output side of your circuit, so it's in series with the triac but not with the control system.