>>>      (when args (setq message (apply 'format message args)))
>>> +    (setq message (propertize message 'face 'minibuffer-prompt))
>>
>> I think it might be a good idea to use a different face for these
>> temporary messages, but you can currently do this:
>>
>> (minibuffer-message (propertize "Foo" 'face 'bold))
>
> So I don't think this is something that we want; closing the bug report.

For consistency with isearch messages that are highlighted with the
minibuffer-prompt face, minibuffer-message needs to do the same.

But since a message is displayed at different part of the minibuffer,
it should be highlighted using a new separate customizable variable
similar to minibuffer-prompt-properties: