Title that is not in anyone's Search feed is found on device via Roku Search.
The fact that this title is found in Roku Search even though it is not available on your channel or any other channel is an expected behavior.
In global search we return unavailable Movies or TV Series if the release or air date is within +-1 year of the current year.
Currently for any non-Direct Publisher partners there is no way to include a piece of content in a feed and still be able to hide it from search.
In this case, because this content also comes from the Gracenote OVD feed, even if you were to remove it from your feed it will still show up in the Roku Search.