My XBox Game Pass has been automatically linked to someone elses EA Account

Thank you Jason for your suggestion.

I need to be careful with my response due to an EA Community Rule: 
"The forums aren't the ideal place for legal discussions and posts including legal chat will be removed"

So I will reply instead with basic guidelines on what is needed to get this issue resolved, so that all the other people who have their accounts linked by EA to other people are hopefully able to resolve it.

I was able to log in to the other person's EA account using my own personal Xbox credentials, which let me view all the personal information on the other users EA account. I could also see that EA had linked four other XBox user's accounts to this one person's EA Account.

I was able to then initiate a chat with an EA Help desk rep from the other person's EA account, I was able to use the personal info I could see in that other account to satisfy the EA reps verification requests and then ask them to unlink the accounts. 

They agreed to do this because I had satisfied their verification requirements, even though I wasn't the EA Account Holder. 

This whole process did take a total of nine different communications attempts with EA Helpdesk staff through four different methods (forums, phone, Facebook messenger and chat) before I worked out the exact details of what needed to be done and found a rep willing to do it.

So when this happens to others, use the above method to resolve it and just keep trying until you get someone willing to fix it for you.

Hopefully, this description is vague enough to not upset the forum rules but contains enough information to allow others to resolve the issue when it happens to them.