If the beacon is covered by bedrock and blocked by the box, it will cause its beacon beam to turn black and is still black after it is stained. If there is surrounding space or transparent box the beam is normal.

Note: this was translated from MC-124270 in Chinese (invalid because it's not English). After quickly checking for duplicates I decided to just re-report it in English (it's not my report). I haven't tested it in-game to see if it actually happens. Just trying to be helpful