I think it is reasobale to show warning even when missing handler is used, since the message warns developers (or translators) some message key are not translated yet.
And, the messages are shown only on development build, so the message have no effect on end-users.

Could you tell the motivation to hide the message when missing handler is used on development mode ?

its ok, just that the doc is saying it will not warn.

missing
Type: MissingHandler

Default: null

A handler for localization missing. The handler gets called with the localization target locale, localization path key, the Vue instance and values.

If missing handler is assigned, and occurred localization missing, **it's not warned**.

#fallbackRoot

Ah, I see it is documentation issue.
Thanks for reporting !