Does polymorph and power word kill instakill? [duplicate]

I have read that the polymorph and power word kill combo can be annoying for DM’s but I’m confused on why it works. In the description for polymorph it states:

The transformation lasts for the Duration, or until the target drops to 0 Hit Points or dies.

Shouldn’t when it dies from power word kill it should revert back to it’s normal form with it’s normal hit points?