If a wither is summoned using the wither skull, the tile limit does not change.

[NamiUni]

There seems to be a missing tile check on the wither summons.

[NamiUni]

There is probably no appropriate API in Bukkit to fix this issue, so fixing it will be difficult.
If there is no indication in the version history that it has been fixed, then unnecessary comments should be avoided.

[FrankHeijden]

To my knowledge there is no event that is triggered when soul sand + skulls are converted into a wither, so getting the exact changes and updating the scancache is difficult. If such an event is added in the future ill be happy to fix this issue.

[galacticwarrior9]

CreatureSpawnEvent.SpawnReason has a BUILD_WITHER reason ("When a wither boss is spawned by being built") -- would that be sufficient?

[FrankHeijden]

CreatureSpawnEvent.SpawnReason has a BUILD_WITHER reason ("When a wither boss is spawned by being built") -- would that be sufficient?

Almost, it's still not trivial to get the locations of the wither skulls / soul sand just from this event. We could assume all of the blocks are in the chunk of the spawned wither but that does not really hold when the statue is built over two chunks.