Prop pixel weirdness

So I have 2 identical boscoyo high res soldier props (150 pixels each). They are both connected at the end of more pixels. Like left rail -> left soldier, right rail -> right soldier. On separate controller outputs.

I've noticed that last pixel of the right soldier doesn't light up. Took it out, tested - everything works. Put it back, test one pixel at time - 149 work fine, last one doesn't. So definitely not output setup issue - all other pixels light up when they're supposed to. Count is also correct.

Ok, maybe last pixel is bad, doesn't read position somehow? Here's the weird part, I flipped left and right and now both light up correctly.

While the problem is solved, kind of wondering what it might be.