Apple account email chanes after accepting invitation

I am trying to add a person to our development portal, the first invitation I send for him to add in our team he receives the invitation properly and is added to our team. But now when I want him to add as a tester to any of our apps, I see the user but his email is not the same that I had added, it shows a different email and the invitation is also not received by the user.

Example:- the initial invite was sent on abc121 @icloud.com and after the user accepts the first invitation and now I want him to add to specific app for testing his email changes to abc @abcxyz.com. How to fix this? the user is unable to test our apps because of this. Tried re-adding the user but that did not help