So, question is whether the issue is with your FC, ESC or Motor..
Easiest way to find out, having a 280 I know there's no reason to have a depinned build, I'm assuming you can move around signal wires on your FC if you have one signal per ESC? If so, change the signal wires around, so that your motors thats not stopping is moved from, say motor 3 to motor 1. Then, give it a test to see how the stop responds? If the stop is still slow on the faulty motor but the non-faulty works perfect in the faulty's FC slot, you can rule the FC out (probably).
This goes without saying but...
TAKE THE PROPS OFF BEFORE TESTING ANYTHING!!!
If it still takes ~20s to stop, it's worth checking your ESC's - what ESC do you use and have you flashed them all to the latest firmware? I would be inclined it might have something to do with setting, e.g. one ESC is set not to have braking on? I've had one motor set on my quad before not break and the others did, which caused some odd behaviour.
If your settings are identical, do you have a spare ESC you can try the motor on or a spare motor you can try your ESC on?
It's possible that you've caused demag issues on your motor, but as you said light crash, it's probably unlikely..
Let us know how you get on!