Yay! We are getting somewhere, guys!)
But there is an issue with the first LED. Its green part always turns on no matter what color I set it to. When I reset the board all LEDs turn off, but when I turn them off manually by sending zeros, first always stays green. In the picture, I made a red to green gradient, you can see that first color is wrong.
@Gordon, the clock is fine, the problem was with the Neopixels soldered upside down. There was a misleading dot in the corner of every LED's footprint. I matched it with the notch of each LED but turned out that dot indicated power pin. When I replaced all the LEDs (they all burnt), the
af_opendrain method worked. I just need to figure out the first LED's issue.