Monster Hunter advancement ,which is defeating all monster, is broken.
The number of defeated monster doesn't increase amount before and after defeating wither.
Perhaps, This bug may affected other monsters.
Linked issues
duplicates 1
Comments 2
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We're already tracking this issue at MC-115750, so I will resolve and link this ticket as a duplicate.
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If you're referring to fact that the Wither itself doesn't affect Monsters Hunted, it probably works as intended as the Wither is a boss, not a typical hostile mob.