Thermostatic Valve Question--is it malfunctioning?

I had a thermostatic valve installed when I remodelled my bathrom about eight months ago. I'm wondering if it is functioning correctly.

Here is what is happening:

I live in a condo on the second floor. My bathroom is directly below my neighbor's above and I'm directly above my neighbor below. When I take a shower, sometimes my neighbor will start a shower after me. When I hear him turn on his water, my shower temperature ALWAYS fluctuates for a few seconds (usually gets cooler), then returns to the temperature it was before.

This never happened when I had my previous valve. I thought thermo was supposed to be better, and it wouldn't do this. So are my expectations wrong, or is it working wrong?

Thank you.

The drop in pressure causes this.