Happy ghasts are ghasts that have been transported to the overworld, which should give you the “Uneasy Alliance” advancement upon killing it.
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Comments 4
Thank you for your report!
After consideration, the issue is being closed as Working as Intended.
Please note, that mechanics of the game may change between updates.
Things such as graphics, sounds, world creation, biomes, redstone, villagers, and animals may not work the same in current versions.
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This might be a feature request.