Illigers don't seem to have the ability to move in water. Illigers spawned in a illiger patrol around a plains village.

The commander and the crossbowman (all of the patrol, no ravager) could not move in the water.

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Illiger raids on a village also get stuck in water when they spawn. (punching them out of water then they run pack in seems to fix the issue). They also don't shoot at me while they are stuck in the water but sill move their heads and make sounds.

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It seems they only don't move when in water is if they spawn in the water.

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Purely speculation here but what if this bug applies to all mobs that spawn in water. Further testing required.