Unicorn HAT - Dead pixel or bad soldering?

I brought my Unicorn HAT out of retirement today for a play, but one of the pixels is playing up. I was coding the board to light up the full 8x8 matix, but it stops at that unlit one in the photo.
Is this more likely a dead or dying pixel or some of my poor quality soldering on the GPIO pins?

If it was a soldering issue non of them would work.
Assuming it only uses the one ground pin depicted in the pinout.

Some Hats do use more than the one ground pin so it couldn’t hurt to have a look see at all the ground pins.

By soldering you mean on the Pi Zero. If you have another Pi that you can swap in that will tell you if its the Pi of hat that has the issue. Assuming its not a software issue, or python file issue.

Unicorn HAT uses addressible LEDs, so if one works they should all work. The one flaw with addressibles is that if one in the chain fails all of the rest of the chain fails, so it seems like that LED is dead.

Thanks for the help. I’ll also try in another board :-)