When loading into the end for the first time, the light from the fire under some of the generated end crystals is bugged. In some cases the light is completely absent, while in others it does not propagate across chunk borders correctly. Reloading the world will fix this.

This does not affect end crystals generated when respawning the dragon, nor does it affect end rods on end cities. Also, the method of entering the end (teleport vs end portal) does not seem to matter.

Seems quite related to MC-264947.