Reproduction link: https://codesandbox.io/s/watcher-clean-up-issue-elnq1

Open the console and toggle the Child component a few times. the onCleanup function registered on the watch is never executed when the component is unmounted, which is one of the expected behaviors as described in the API reference:

The cleanup callback is called when:

the watcher is about to re-run
the watcher is stopped (i.e. when the component is unmounted if watch is used inside setup())

I also tried to manually stop the watch and the onCleanup callback doesn't run then either. It does run however before the watcher is about to re-run, as you can see by typing something on the provided input.

It could be however that I'm misinterpreting the expected behavior.

Thank you!