Series X showing and listed as an XBOX One on registered devices.

You mean removing 360 registered consoles serials from the account? Yes! Both my 360 aren't listed anymore on my account as "registered devices". I think they did it once the 360 hardware support was gone. I still can see my One X and Series X registered.

So, I just tested this to see:

I registered my Series X controller.

Sure enough, it said it was registered but wasn't showing up

I then clicked "see details" on my Series X entry which took me to my Series X

I then clicked "All Devices" to take me back to the previous screen and the controller then showed up.

So, not entirely sure why it won't work for you

After I registered the controller on the Microsoft website it showed on my devices list with a one year warranty attatched to it.

Hi

If you register your Xbox One X to your account using
this link when you then select your Xbox One X console from the list it will then show the warranty expiry date. If you wish it may also be possible to purchase

Microsoft Complete for your Xbox One X that will provide you three years service coverage on your console for a very good price.

The device might still be registered on your Microsoft devices list. Check on here

https://support.microsoft.com/en-us/devices If it is, then un-register the device, and hopefully it should no longer show up on your list of devices to install to.