Iām also experiencing this with Bring Home the Beacon in 1.21.5, and, through testing, agree that it seems to be caused by the Beacon being placed at a low Y-level and/or the beam intersecting transparent blocks. Even without moving the player, simply breaking and replacing the beacon will only sometimes give the advancement when the beam appears (tested using a repeating command block to instantly revoke it).
J Cardwell
Iām also experiencing this with Bring Home the Beacon in 1.21.5, and, through testing, agree that it seems to be caused by the Beacon being placed at a low Y-level and/or the beam intersecting transparent blocks. Even without moving the player, simply breaking and replacing the beacon will only sometimes give the advancement when the beam appears (tested using a repeating command block to instantly revoke it).