If a user updates his email address - and then pays through the same gateway as before - to prolong his account - the old email address will be used there. This is wrong and leads to invoices not arriving or similar.
While the email address should not be changed for ongoing subscriptions - for a new payment the new email should be used - not the old. Often users don’t even have access to the old email anymore. Stripe/paypal integrations like invoices need to have the new address.