This might be something we can change but it isn't necessarily a bug.
The examples that
do work only work because we canonicalise the request URL which is an opt-in thing and not necessarily a thing we do as standard.
It's also not technically directly related to route filters.
For example, I can link to this thread with a changed title portion:
https://xenforo.com/community/threads/maybe-a-bug.177818/
But when you visit the link it is redirected automatically to:
https://xenforo.com/community/threads/route-filter-for-misc-contact.177818/
That's URL canonicalisation and while it does happen if we opt-in to it for route filtered roues, it isn't exclusively tied to route filters as demonstrated above.
We don't perform URL canonicalisation on the contact page which just means it's something we haven't explicitly opted into there but it isn't necessarily a bug.
Indeed there may be other pages where we haven't opted into it either and those, equally, wouldn't necessarily be considered bugs either but we will give it some thought.