2nd master's degree in "slightly-different" field? data science and machine learning specifically
I don't know of any generic rules which forbid this. Indeed taking a masters degree after another higher degree is an accepted, although uncommon, method to switch career paths fairly deep into one's education. On the other hand, as you note yourself, that's not really what you are suggesting here, and any application is going to have significant barriers to overcome, firstly as to how you'll be funding this (masters courses are expensive, after all) and secondly as to why you aren't following the more natural career path of moving on to a Ph.D. if you want to continue your studies. Which isn't to say that a sufficiently strong narrative wouldn't manage to get you an offer if numbers weren't limited and you could explain your funding.

(It's worth noting that while the University probably doesn't have a rule against this, many potential sources of funding do)