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.
Environment
OS: Windows 10 Java: Version 8 Update 111 (build 1.8.0_111-b14)
Duplicate of MC-114881 - this will be fixed in the next snapshot.