When you see a zombie in the river and when you go out of its sight and wait for a few minutes and when you come back there is full durability whole set of gold armor just bouncing on the river

The zombie was wearing that armour set, when that zombie turned into a Drowned it dropped all of its armours with full durability, then because you went away from the Drowned it probably despawned.

Thank you for your report!
We're actually already tracking this issue at MCPE-33140, so I will resolve and link this ticket as a duplicate.

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