The bug
Wither bosses can shoot blue wither skull if no enemy is around and there is enough space. The value if a wither skull is blue or not is not saved in the NBT data which means it loses this after reloading it, you cannot summon them in and you cannot test for them.
The reason
The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.
The reason why this happens is because the field net.minecraft.entity.projectile.EntityWitherSkull.INVULNERABLE is not written to the NBT data and cannot be set based on the NBT data.
Note: A tag for this field should not be named "Invulnerable" as this value is only used for less destroyed blocks when it explodes and for rendering. The behaviour of the "Invulnerable" tag is not affected by this.
Related issues
is duplicated by
relates to
Comments
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marcono1234
Updated the report
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Still an issue in 18w14b.
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Confirmed for 1.13 pre 2
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For reference: bedrock editions save it under a separate id: wither_skull_dangerous, so that’s likely a better fix (do note that saving it as dangerous_wither_skull, does make more sense).
ampolive
Can confirm in 1.17.1.
muzikbike
Likely relates to MC-172738
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Same issue, yes, but the code to save field for these entities is not at all related, nor are the entities themselves related.
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This is still an issue in 1.19.3. For context, this was reported 7 years ago.
clamlol
Affects 23w33a. Here are some repro steps:
How to reproduce:
Get to a safe area in a creative world (eg. above the nether roof)
/summon minecraft:armor_stand ~ ~ ~ {Invulnerable:1b,NoGravity:1b,Passengers:[{id:"minecraft:wither"}]}Wait for the wither to shoot its blue skulls
Reload the world
Expected behavior: The skulls would remain blue.
Observed behavior: The skulls are now black, and some slight jittering due to MC-80142 can be observed if the skull flies far enough without hitting anything.
Nathan Adams
please submit one issue per ticket, there are actually 3 diffrent issues all relating to each other