Subscription lookup tool doesn't always find subscriptions
The subscription lookup tool only finds subscriptions if the email address the customer provides is the main account email. If they supply the billing email, then the lookup tool doesn't find the subscription (apparently, see ticket #1315676 for reference)
The lookup tool should find the subscription no matter which of the possible email addresses are provided.
Right, so the billing can be different to the main WooCommerce.com account email. That sub search is by main account email only.