Deleting a restricted page doesn't correctly apply restrictions to child pages

Steps to reproduce:

• Create a page "Parent page", with a child page "Child page"
• Add page restrictions to "Parent page"
• Delete "Parent page"

Expected behaviour:

• "Child page" is still restricted, but with a copy of the page restrictions from "Parent page".

Actual behaviour:

• "Child page" is restricted by virtue of inherited permissions from "Parent page" in the trash
• When "Parent page" is purged from the trash, "Child page" is unrestricted.

Workaround:

• Move "Child page" to a different parent, then apply restrictions manually to "Child page".