That setup should have worked fine. You’ll need individual testing to determine where the problem is or was.
In my experience UV leds turn black on the die when overheated. Its easily seen by eye.
Another possibility is that it had a bad reflow. Sometimes when reflowed there is only a tiny bit of solder that connects the led to the mcpcb. It will work fine until it gets hot or enough current is applied to pop the fuse (the thin connection of solder) so to speak. Sometimes they will even work fine at low currents but at high currents it just shuts off, which I think is more heat related in a bad solder joint between the led and mcpcb. Just another possibility to check.
Once you get it tore down let us know what you find.