The Bug
When an End Gateway is in the Nether, the beam renders pure red for a bit instead of purple
Reproduce
Go to the Nether
Do /setblock (coords) end_gateway
Look at the beam
❌ Notice that the beam renders red instead of the purple
Note
This doesn't seem to happen in 1.16.5. Provided an image for 1.16.5 to show this
Can confirm. Relates to or even duplicates MC-222104.