Logic level shifting
THe WS2815B operates on Vcc= 5V supply logic levels from 0.3 to 0.7 x 5V  not the LED 12V supply.  This means it is similar to the 5V CMOS Schmitt trigger input.

The LED supply is called \$V_{DD}\$ and intended for automotive 12V supply voltages or +9.5~+13.5

The Din Data port operates off \$V_{CC}\$ which is internally regulated and brought out for a Cap to added as an option.  Check Pin1. It must be 5V.

Pins 3,4,6 are 5V logic level signals.  BI on pin 6  is grounded only on the 1st chip.

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The datasheet is a bit unconventional lacking in some details, so this confusion is expected.

They could have included something like this.


Perhaps they assumed too much.

The chip ought to have diode clamps to internal 5V on the input from the 10k Pullup you used.

Your design looks ok except the pullup should be to pin 1 = 5V 

Depending on the cable capacitance the 10k may need to be lowered towards 1k.



I added in BLUE