The magenta end gateway beam for initial placement uses 256.0D - y to determine its height. While it looks like this would calculate a height to 256 blocks, y is actually a relative distance between the block and the player, not the block's position. This causes weird height issues, because the height can be negative if the player is below the gateway; if they are over 128 blocks below things get very odd.

To reproduce

1. Run /setblock ~2 255 ~ end_gateway to create an end gateway block at y=255
2. Enter /data merge block ~1 ~ ~ {Age:100} into the command block.
3. Change the command block to a repeating, always-active command block.
4. The beam should now be visible.
5. Start flying down, while still looking up at the gateway (otherwise it will vanish due to MC-136756)
6. At y=128 it'll pass you and start disappearing.

Fix

Fixing would require either specifically using the block entity's position to get the Y coordinate, or just hardcoding a fixed height of 256 in TileEntityEndGatewayRenderer.render. Note that when using the Y coordinate, it would stretch to y=256, but not down to y=0; additional code would be needed to make the part below go that far down.