Chemists create the brightest-ever fluorescent materials

Probably not. Fluorescence occurs on a nanosecond timescale. Phosphorescence (what's used to make white LEDs) can have timescales on the order of seconds.

What's far more likely is that it's a capacitor discharging.

The slow fade is because there is a large electrolytic capacitor in the power supply which retains a small amount of energy when the power supply is unplugged. This slowly drains (depending on circuit) and is why it sometimes take up to a minute to see the LED fade. When repairing or modifying power supplies it is important to be aware that capacitors may still be storing enough energy to cause a shock.

Fluorescence means emitting light after absorbing it- higher fluorescence means that less energy is lost.

Fluorescent lightbulbs drop ultraviolet light down to visible light typically via a phosphor coating. White LEDs are also often phosphor (or similar) based.

Note that in neither of these cases is the phosphor considered a "dye". I suspect that any application of a fluorescent dye on the LED itself would still cause a fairly significant luminosity loss and buildup of heat compared to simply coloring the plastic or glass enclosure that the white LEDs are built into.

I am not a materials scientist or engineer, just a person who likes to wildly speculate on the internet when I don't get enough sleep.

Hands for scale https://scx2.b-cdn.net/gfx/news/hires/2020/chemistscrea.jpeg

There are hands underneath the objects in the first picture...