If it’s any comfort knowing, mine is the same on Xbox too. I suspect something is broken on their side. I don’t recall properly, but when they first setup the IOI accounts there wasn’t an option to sign-in via Xbox. I think the only option was to use an email address and I’m assuming you also created an account it in the beginning too. It looks like they’ve got the option of signing in via multiple gaming platform accounts now. So for people who joined more recently it probably makes use of that platform link.
My only encouragement is that my IOI account correctly lists all my purchased Hitman content for Xbox and I can see in the Hitman 2 game under Account on Setting that it is registered with the same email address as the website. So I’m hoping that is enough to ensure progress carry-over.