EDIT: Perhaps at the time this question was asked the search wasn’t working for the email which isn’t the primary in the linked account but I have just tested this and it appears to be working now.
@MattLiu it is possible to find the user from the linked accounts email address using identities.profileData.email query parameter. You may find different samples for queries on this link..
Here is a sample for searching firstname.lastname@example.org in the linked identity:
curl -H "Authorization: Bearer MNG-API-TOKEN" https://tenant_name.[region].auth0.com/api/v2/users?q=identities.profileData.email%3A%22test%40test.com%22&search_engine=v3
@a2345sooted this also seems to be fixed. For a linked account , when I try to signup with an email which exists as a secondary identity for another user on the same connection, signup is failing with an error that the user email exists.