Why doesn't sign-in with apple work with Pokemon GO?

Everytime I try to use Sign-in With Apple for my Pokemon GO account, there is an error:

Account Already Exists

This account is already linked to an existing player.

This is completely false or have I been hacked? I can still login to the same AppleID that claims is already using the sign-in on another player's account, so I removed the Pokemon GO sign-in and try using it again and I get the same error and I see it is not connected. When I logout of the same AppleID and login a short while later, I see that Pokemon GO is now using my AppleID sign-in. I can remove the access and repeat as though I am doing wax-on, wax-off and nothing changes. Is there some security setting I need to change to allow for Pokemon GO to use this account or is there a policy at Apple that doesn't allow for anyone to Sign-in with their Apple account?

Thanks!

MarioJBru