When a zombie wears armor underwater it sometimes do not drop the item they were wearing when they become Drowned. I searched in Minecraft Gamepedia the Drowned do not wear any armor at all
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They should always drop armor - are you sure you havent missed it?

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We're tracking this issue in MCPE-89100, so this ticket is being resolved and linked as a duplicate.
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They don't always drop armour, so I'm pretty sure this is just a thing. Could you send a video?