An observer is considered a transparent object but for some reason, the beacon does not activate. The beacon behaves as intended when pistons, glass, or slabs are above it and they are all considered transparent objects.
Comments 3
Please provide a reference to an official source (not the game Wiki) that says that beacon beams will not be blocked by observers and glowstone. Since you can't see through these blocks, they would not be considered "transparent".
Cleaning up old tickets: This ticket had been set to 'Awaiting Response', but has not received a response from the reporter (~3 months+) so is being closed as Incomplete. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines which includes steps to reproduce the problem.
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Update: I tried it with glowstone which is also a transparent object and the beacon didn't activate either, so it might be multiple transparent blocks in the game