I was always taught growing up that "next" Monday is the one after "this" Monday which is the first Monday you will encounter moving into the future.
I was surprised as an adult to discover that this is not universally held. Since it's pretty obviously ambiguous, I would expect my voice assistant to ask for clarification on which one I meant, and maybe learn from that if it's the kind of assistant which keeps user profile data (which is probably all of them).
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...”
In German* it depends on the tense of the verb. “On Monday I went to…” refers to the most recent Monday in the past. “On Monday I go to…” refers to the closest Monday in the future. Same for “This Monday…”. “Next Monday” refers to the second closest Monday in the future.
I was surprised as an adult to discover that this is not universally held. Since it's pretty obviously ambiguous, I would expect my voice assistant to ask for clarification on which one I meant, and maybe learn from that if it's the kind of assistant which keeps user profile data (which is probably all of them).