Aliens. Communicating to the Mother Ship. Save yourself while you can...

Are they on the same circuit breaker?

Not sure, but lets assume they are.

It may be related to your trash can, or maybe your neighbors (the ones with the spiffy new trash can) are aliens. Do they have domed hats?

Is the trac lighting low voltage? 120v. transformed to 24v as an example.

Might be a surge affecting the transformer or the wiring. When electricity is shut off and turned on in a circuit, there is a magnetic wave created(this is how alternators create electricity). Check the wiring to see if the wiring of the two circuits are close together(might need an electrician).

You need to talk to the electrician that wired the house ASAP! I'm no electrician but it could be a wire crossed over another wire somewhere in the wall that shouldn't be.

LED lights? Electronic dimmer? Nothing would surprise me.

The track lighting is LED line voltage, and it has a leviton zwave plus smart dimmer switch controlling it. The switch for the fan is just a regular on/off paddle switch.

Change out the dimmer switch. And put on your foil hat just in case.

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LEDs can light (if dimly) with a remarkably small amount of current. When you shut off a motor, it can generate a spike in the cable feeding the motor as the motor's magnetic field collapses. Put a cable to the light right next to the one feeding the motor, and some inductive coupling between the cables is possible. There's your flash (maybe).

Try connecting an appropriately sized MOV (surge suppressor) across the motor terminals, and preferably add 2 more, one each from hot and neutral to ground.

We have the same phenomenon with an LED light and the exhaust fan in our master bathroom, new build. The builder explained the cause similar to that stated by DavidR.