Sorry, it's difficult to describe this issue as a simple summary. To see it for yourself, turn suggestions on and type /give @p milk. Obviously you are trying to type milk_bucket, but the suggestion acts as though you have typed mi as in minecraft. It shows necraft:milk_bucket as the rest of the suggestion. Items who don't share the beginnings of their names with minecraft work properly.
Environment
Windows 10
Note that this is no longer observable in 18w06a because the whole tab suggestion is now displayed (the yellow text), however this may still cause other issues in the future.