zombies don't become the drowned mob when they die

This is probably not a bug, just an incomplete feature. From the change log: "These features are incomplete and are not representative of final gameplay but feedback is absolutely welcome! These are also not all of the Update Aquatic features coming to Minecraft and more is on the way."

When Drowned were introduced to Java snapshots, they also didn't spawn from drowning zombies at first.

Same but like u said referring to what the update stated

Thank you for your report!
However, this issue is a Duplicate of MCPE-31531.

If you have additional information, please add it to that report.
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