Short: Use jlcpcb’s “Standard PCBA” assembly option with 240 reflow temp when using WS2812B LEDs. Explicitly check datasheet reflow temps being used by assembly service.
Long:
Sharing incase someone else here is playing with LEDs/WS2812B’s and using jlcpcb assembly service…
Fixed bunch of code and design issues on my part. But I couldn’t understand/explain this…
Until now…
Just realized jlcpcb PCB assembly service, by default is “Economic PCBA”. That service lists the reflow temperature as 255.
However WS2812B datasheet reflow peak is 240 
So, guessing many of the LEDs were overly cooked.
Fortunately jlcpcb assembly have a “Standard PCBA” that you can optionally select with reflow temp of 240. So, will explicitly use that next time…
Lesson… RTFM, read the datasheets, read the service details. Read everything with the same diligence you’d give to ensuring grub screws are tight.