How to reproduce

Use the following command

/setblock ~2 ~ ~ end_gateway{Age:4780L}

→ The beam appears one second (20 ticks) after the end gateway was placed, despite no entity being teleported

Workaround

tryashtar mentioned a workaround for the time being:

Set the end gateway's Age to a negative value for example -9223372036854775808L since that is the lowest long value possible and is about 14623560433 years in the real world.

Note: Be aware that this workaround will also remove any beam occurrence even if an entity enters that gateway
(as long as the gateway got a negative Age value).

If you do need a beam whenever an entity enters the gateway, but want to additionally prevent the currently periodically recurring beam each 2400 ticks, set e.g. a command block contraption with a timer that resets the gateway's age to 200 (timer at maximum at 2199 ticks = 1 minute and 49,95 seconds realtime).

Code analysis

See this comment.