Fellfromtree mentioned this thread & problem over in the BLF17DD Info Thread - Reference (post #142) and I thought I’d chime in. Fellfromtree mentioned a suspicion that the gate drive resistor might be incorrectly spec’ed, but I don’t think that should be the problem. AFAIK the gate drive resistor is simply there to protect the MCU from a potentially heavy drain on it’s little 20mA-per-pin sourcing ability - NOT to protect the FET from anything at all. Besides just getting a bad FET, I think the most likely thing sounds like a sudden open circuit during operation.
Offhand I’m not sure if turning off the tail clicky should be able to cause a voltage spike, but disconnecting the LED definitely should be able to do that. (Due to the inductance in the wires to the LED.) Here it sounds like the LED was already securely wired in, so I suppose that we can assume the LED was not suddenly disconnected it. In that case I guess the tail clicky did it? Hmm.
BTW fellfromtree… you have confirmed that the LED is OK, right? EG at some point after this incident occurred you wired this LED up such that you saw that it is capable of lighting up at the normal maximum which you are accustomed to?