Hello, I came across this post by chance, the recurring beam each 2400 ticks occurs since 1.11 snapshots, I happen to host and update the bugpost for it on Mojang's bugtracker Mojira: https://bugs.mojang.com/browse/MC-107824
You can find a workaround for Minecraft Vanilla Creative (via Commands) in the bugpost description, just click the link above and read.
As for now it's not confirmed by Mojang whether or not this is "works as intended" due to the added portals in The End on the outer islands (some sort of recurring "I am here" beam as a player help in Survival), or if it is actually a bug.
For mapmakers, this should surely be fixed, and if it's "works as intended" for Survival Players, I hope they'll add an additional tag or the likes for Creative/mapmakers, so that periodical beam does not occur without having to add an age-reset-timer or without setting a negative age.
I hope this helped