From this github help page
When you delete a public repository, one of the existing public forks is chosen to be the new parent repository. All other repositories are forked off of this new parent and subsequent pull requests go to this new parent.
What happens if the chosen fork is some commits behind the parent, will the chosen fork automatically receive a pull request for the changes?
If you mean "behind the parent who just got deleted", no since it is just deleted.
It does not appear that the new parent benefits from a
fetch old_parent before
old_parent is deleted (if that was the case, you would still see the old pull requests submitted to that old repo as remote branches in the new repo)
That is why the help page says: "subsequent pull requests go to this new parent."
Unsynced commits to the old_parent repos will then be lost