>>>> 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.
>
> [...]
>
>> +    (setq message (apply #'propertize message minibuffer-message-properties))
>
> But this will overwrite the face properties the caller has set, right?

This patch checks for existing face properties to not overwrite them: