In the snapshot 17w13a for Minecraft 1.12 I made a beacon 3x3 and a full powered beacon both using diamond blocks. After I built it I noticed the beam doesn't appear and there is nothing blocking it.
MC-115097
Powered beacon doesn't emit a beam
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Ghost8797
(Unassigned)
Unconfirmed
block, rendering
Minecraft 17w13a
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Duplicate of MC-114881 - this will be fixed in the next snapshot.