Must be getting too old for this ...   I guessed (correctly) that it had to do with type-constraints and with coercions, but simply overlooked the importance of that flag.   I guess I decided too-quickly that maybe it was looking for a particular type-definition and not matching the coercion specification, instead of not attempting any coercion at all.   Merely specifying a coerce rule doesn’t cause it to be invoked.   Sigh.   Shows what ’ya get sometimes, for chasin’ them red herrings and jumpin’ to conclusions ...

Thank you, sincerely, for pointing out these documentation references to me.