I filled T285748: 'relative' => true makes autocomplete use the main namespace, regardless what 'namespace' says about a bug in mediawiki's special page systems. Since I currently don't have enough energy to look at it in depth, I think we should remove relative => true from special:Editgrowthconfig, and use namespace prefixes instead.
As soon as the task is fixed, we can revert that workaround.
When you try to add an (existing) template if there's a main space page starting with the same title, it won't be possible. See below:
I'm trying to add template called পরিষ্করণ to the form, but that's not possible, because there's an article starting with the same prefix (and due to exists => true, it shows no redlink autocomplete I could use).
(screenshot taken on bnwiki)