Well, at least you’re making progress and getting to the bottom of things.
I’m not sure the product you linked as “spec sheet” is this one… it clearly shows 3 wires and claims a WS2811 IC.
Just wanted to point out this thread as well; similar in that it’s a 12V, dual-path NeoPixel, but spec sheet says to ground the backup input. I see from your picture it looks like DAT is conected to two of the 4 wires going to your LEDs.