Subpages don't inherit permissions from parent pages (see comments for solution)

We are currently experiencing a serious issue with page restrictions. We have pages with restrictions, that have sub pages, which were created by users, that were deleted from the user directory in the meantime. These root-pages have read restrictions, set for a particular group. However, these sub pages are listed in the activity stream and are also discoverable for users through global search, that are not a member of the particular allowed groups, set in the page restrictions.

The global search will display these restricted articles and unauthorized users can fully open these pages. Opening these articles does also cause a "500 Internal Server error".

Note: the affected pages/ subpages were migrated from version 4.x to 5.x.