I was taught that there are two interpretations, and therefore it's always a good idea to ask for clarification. At least when interacting with humans.
So, what you said is exactly right: Siri should get clarification. And also when stating things in the form of "next Monday" Siri should clarify what is meant, or just avoid the form altogether by instead saying things similar to "this coming Monday" or "the Monday following this one."
In an ideal world Siri would ask you once then learn your style. I would also like to offer a correction. I don’t want Siri to always ask “Did you mean...”
So, what you said is exactly right: Siri should get clarification. And also when stating things in the form of "next Monday" Siri should clarify what is meant, or just avoid the form altogether by instead saying things similar to "this coming Monday" or "the Monday following this one."