Since Version 1.8 (a long time ago ...) beacons beams do render differently.
Before it rendered for 256 upwards.
Now, it renders only until y=256
So, if you place a beacon near skylimit, its beam is very short.
MC-106327
Beacon beams do render differently since 1.8
Resolved
Duplicate
0
Michael Campanelli
(Unassigned)
Unconfirmed
Minecraft 1.10.2, Minecraft 16w32a, Minecraft 16w32b, Minecraft 16w33a
Retrieved
You may have searched around a bit, but had you searched 'Beacon Beam Height', you would've noticed this is a duplicate of MC-67975.