Say for example you have domain.com/page/
You set the rule
Source: /page/
Match: Url & WordPress Page Type
When matched: Redirect to URL
Destination: domain.com/new-page/
Say for example /page/ was delated prior to applying the rule the page should now return a 404 error after applying the rule it should now redirect all visitor from /page/ to your destination url domain.com/new-page/. The thing is it doesn’t redirect it but instead it just showed the 404 error page as if the rule wasn’t applied.
However if you were to change the Match Type to url only, when you visit the page it would actually redirect all users.
Which now leave me to conclude that the plugin isn’t actually detecting that the page is actually returning a 404 error.