MC-307449 (AKA, the Spawner Bug) needs to be fixed

MC-307449 should to be a priority fix, due to its disproportionate impact on survival map making communities, especially Complete the Monument map making communities. This is the bug which causes Mob Spawners with custom spawn rules to be unable to spawn mobs when underground in modern MC Java versions. Because custom spawners are so important to making these sorts of survival maps, these communities have essentially been trapped in the version before this bug was accidentally introduced (version 1.21.11), leaving the people in these communities unable to innovate with recent features added in the newest versions.