Can you pay to use the app for only one or two months? Or does it requiring subscribing and cancelling?
If you purchase an in-app subscription through the App Store with your Apple ID, the only option is a yearly subscription.
If you purchase a subscription directly from Microsoft's website, you can purchase either a yearly or monthly subscription.
The yearly subscription costs the same in both cases, and is cheaper unless you only want to use the app for a few months.
Is this through in-app purchases? It would seem counterintuitive to let Apple take 30% from the yearly subscription if they've gotten away with distributing outside the app store for so long. (but they may have a special revenue share agreement)
If Microsoft acquires a new user and subscription through the App Store then Apple is entitled to their cut. If Microsoft acquires a new subscription organically outside of the App Store then Apple is obviously not entitled to a cut.
If Microsoft doesn't want the ability to acquire new subscriptions through the App Store (and pay Apple for it) then that's their prerogative, but right now that's not the case.