The end gateway beam is culled when the source block isn't visible, similar to what used to happen with the beacon beam (MC-68247) although slightly less aggressive.

To reproduce

1. Run /setblock ~2 ~ ~ end_gateway to create an end gateway block.
2. Place a command block next to it.
3. Enter /data merge block ~1 ~ ~ {Age:100} into the command block.
4. Change the command block to a repeating, always-active command block.
5. The beam should now be visible.
6. Looking forward, fly up. After a few blocks, the beam should disappear.
7. When looking down, the beam will reappear.