Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
First off, I'll say that I have two Microsoft accounts, a web-version provided through work, and a personal paid subscription as well.
In my personal email account, that is not Google and on a secure server, I do not receive any spam mail here, with the exception of spam mails all originating from "onmicrosoft.com"
This is clearly a microsoft issue, and they clearly have no intention of resolving it. Their feelings about their products, services, and customers is also quite clear. They don't care.
The obvious solution is to block all emails coming from an "onmicrosoft.com" account. You may miss some legitimate emails, but as another user stated, this will just be an opportunity to explain to someone what the problem is. Eventually microsoft will lose all users to this platform, and maybe then they'll start to care.
On another note, I had filed a BBB complaint against them for another issue. This went around tech support for a month or so. From this experience, I can tell you with absolute confidence that they do not care about their customers.