If a player rides on a nautilus and attacks an untamed nautilus, the nautilus will try to fight back against the player; this is completely normal.However, if you slightly control the player's ascent when the nautilus rushes toward the player, the rushing nautilus might hit the nautilus that the player is riding, and multiple impacts could kill the nautilus.
Under normal circumstances, nautiluses will only retaliate against players. When retaliating, they should not attack other creatures that come into contact with them, because those creatures did not attack the nautilus. However, for some reason, a player's nautilus may still unintentionally harm other creatures, even though those creatures were not attacked at all.
The two bugs I submitted last time prevented you from tracking them, so I revised the description and resent it.（MC-302996，Returned）
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After consideration, the issue is being closed as Working as Intended.
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This does make sense though as skeletons can accidentally hit other mobs when shooting and the nautilus isn't an exact “deal damage to target mob like a zombie“ it is designed so the player can ram it into whatever mob they like.
I may be being annoying with this, but even if it is unintended it could still be fun to dodge a nautilus/zombie nautilus and have it accidentally hit drowned mobs. I wouldn't mind if this got left in personally.