I'll link the wiki as it may be helpful for the thread: https://ark.wiki.gg/. We use the main namespace for official game content, and a "Mod" namespace for unofficial modifications, all following a format of a mod's main page at "Mod:modname", and mod's content as sub-pages to that main page. For example "Mod:ARK Additions/Acrocanthosaurus".
In my expectations, typing "Acro", "Acrocantho", or the full title "Acrocanthosaurus", in the mw-head search bar would suggest the article that's in the Mod namespace. We have no other page titled Acrocanthosaurus in any namespace (ignoring files of course). However, there are simply no results returned at all.
To get the suggestions, the reader has to type the mod namespace prefix and the mod's name. "Mod:ARK Additions/Acr" returns valid suggestions. There's no "partial" completion, the prefix must be complete and without typos. And that's not very intuitive or useful.
Regular Special:Search already handles this well [enough], and our mod namespace is weighed below main.
(I've put "enough" in brackets, as searching for "acrocanth" in Special:Search yields no results until a wildcard is added to the end. I'm not familiar with Cirrus's configuration though, so not sure if there's a setting to alter the behaviour so search acts as if there was a wildcard at all times. However, this is not related to this thread.)