No surprise here. Apple's position, which I expect they'll likely eventually prevail on, is that none of Masimo's relevant patents are valid and they should have never been issued. Why pay money to license an invalid patent?

I don’t really know any of the intricacies, can you elaborate on why they are not valid?

The short version is that a lot of patents were issued in the 90s and early 2000s for "inventions" that actually already existed "but on a computer!" After a lot of legal wrangling the standards got stricter and these never-should-have-been issued patents have been systematically invalidated, though it's a one-at-a-time process. I think Masimo originally claimed infringement of a dozen patents. From memory, it's now down to two patents that have not been entirely invalidated, and I think even those have already been carved down to remove most of the claims. So basically there are two half-patents left to litigate and Apple thinks they can finish those off as well.

Apple is more than happy to leverage such patents when it sees fit. They’re just throwing a tantrum because they finally ran into a small corporation that they couldn’t intimidate.

I don’t see how intimidation has anything to do with it. If you think the patent is BS and you have the financial resources to actually fight it, it’s good to fight it. It’s better for everyone when patents that shouldn’t have been issued are invalidated.