Steps to reproduce (a video is attached):

1. Enter the end and go to a spot with light level 0.
2. Optional: Set Brightness to Moody to see the bug better.
3. Place obsidian into the ground.
4. Place an end crystal on top of it so fire also spawns on top of the obsidian.
5. Look at the end crystal and wait.
6. Sometimes, the fire extinguishes and then quickly relights itself, producing a pronounced flickering effect.

Note that this is not MC-279068 (also visible in the video), because the flickering has a different cause here and is much more severe.