My 36" does that. Sort of interesting as well as how long it stays lit after it's unplugged.
I figure its burning off whatever power is stored in the driver, but hate that it gets brighter. I'd rather see some other sort of bleed off or taking longer to fade off.
My dimmer doesnt seem to work through the full range of the dial either. it quickly goes from the min to max, while still allowing me to turn the dial considerably more.
other than these minor issues, the amount of oxygen bubbles in the tank increased a whole lot after less than an hour with the LED on top. So it's definitely powerful enough for a planted tank. I'm really looking forward to the growth.