Electrical – New LED flood lights give off quick flash when turned off

It's not the wiring. It's the LEDs themselves.

LEDs have a characteristic delay when turning them on which may be more than you're used to from a lifetime of incandescent bulbs.

Swap the LEDs from one position to another, and the problem should move with the bulb. Replace the LED with the equivalent incandescent (temporarily) and it should go away entirely.

Unfortunately, home LEDs are still relatively new and they suffer from both this characteristic delay --- which may vary from brand to brand -- and manufacturing variation --- where bulbs from the same manufacturer may vary, even within the same batch.

The only way to avoid these issues is to closely watch online and professional reviews (from trusted sources) to pick the better-performing brands; return any product to the store that is an "outlier" in the same room -- or, wait another few years for LEDs to mature (I'm serious) and just live with it for now.

It could be the bulb circuitry going bad. LED lights are different in that most of the "bulb" is actually voltage conversion and a simple circuit system to regulate the LEDs.

This is just an educated guess, but what it sounds like is there's a capacitor inside that still has power in it. The light bulb itself probably has something that senses when the power is turned off and turns the LEDs off via software, letting the capacitor just lose voltage through attrition. If that part of the bulb stopped working, that capacitor could still power the LEDs for a few seconds before running out of power.

You can test this theory in a regular lamp. Just turn it on for a minute, then turn it off. I don't know that it's dangerous to let it keep on doing this (it's probably been doing it like this all along, just with the cutoff working), it's just not expected behavior. If it doesn't overheat in the lamp I wouldn't worry about it.