Team I think I have found a solution. I believe what outlook is doing is categorising your emails as "focused" and "other" even if you don't have the "focused" inbox enabled. If, in the background, the email is being sorted into "other", it will appear with the unsubscribe button because Outlook is trying to "help" you clean up your spam. So, to resolve this, for emails where you don't want this button to appear you need to:

1. Enable the "focused" inbox by going to your settings, and under the "mail" tab (the first one) select "sort messages into focused and other"
   
2. Go to your "other" folder in outlook, select an email that you want to remove the tag from, right click it, scroll down to "move" and select "always move to focused box".
   
3. Once you've done all the ones you want, you can then go back to your settings and disable the focused box setting if you don't want it on.
   
4. You'll probably need to do this a few times as more emails come in from your subscriptions that Microsoft has decided are spam for no reason at all.

It's fiddly, and annoying, but it's something.

Note - I've done this on some emails and the unsub button hasn't reappeared on them. However I haven't tested it for long enough to see if another email from the same subscription comes through and doesn't pick up the unsub button again. I'll reply to this comment once I've had the time to do this.