Hi @Leo_Bettinelli , Glad to hear that you got a solution but I can’t help you much with a logical explanation.
That 1st pixel does absolutely nothing different than all the other pixels.
The only thing that comes to mind is that the 1st pixel is the only one that receives data from your microcontroller. All the other pixels get a data stream that gets ‘reshaped’ by that 1st pixel.
Actually all ws2811 reshape their incoming data, not just the 1st pixel. Have a look at this very nice description of how the ws2812 deals with incoming data…
When you mentioned the removing of that 1st pixel and the problem then moving to the 2nd, I did give some thought to the possibility of bad data from your microcontroller or a bad data connection from it but because of the joint soldering fixing the problem, It did not make sense. Also that 1st pixel seemed to work OK with the data it received so it should be good data !?
Could you have simply corrected a bad GND problem by inserting that 1st pixel from another strip ??