The sentence as read implies "we have no other lessons that day besides maths".
Your sentence could be (and I think would more usually) be read as implying "we don't have maths lessons on any other days of the week". I think it would be more like у нас математика только в понедельник.
Does that help? I'm a native speaker of English, and though I don't really know the technical reasons, that's how I'd understand those sentences if someone said them to me. I think "We only have maths on Monday" is slightly ambiguous, but the first is a better translation.
I think that this should be accepted, still. "We only have maths on Monday" is absolutely ambiguous, so it could mean equally either. For example, for me personally, if someone asked me, "Do we have Science tomorrow?", I would respond, " No, we only have maths on Monday". Maybe that's slightly incorrect, but it's still common-enough usage that it should be accepted here, imo