Update PayPal email address
I have found that once a PayPal email address has been captured, your system won't let me update it, even if I delete it and replace it with completely different information on PayPal's end. PayPal email addresses should not be immutable information. Our organisation has had to change the address we use for PayPal due to receiving spam on older address a number of times. It will happen again. You will need to either sync your data with PayPal, so it is updated and correct, or let customers change it for themselves.
Andre J commented
I received confirmation from the support team that the cause of the issue has been identified and a once-off fix has been applied, relevant to my instance. Further work will probably be needed to support others who will inevitably encounter similar meta-data issues in future.
Here is the feedback from the support team on my instance:
"This is indeed related to metadata on our end storing the old e-mail address and when linking it was using the old email address. We've gone ahead and removed the old email address internally. Please proceed to re-add your new email/account for paypal. Reach back if the issue continues."