If the user tries to reset their password on a tenant where they don’t exist, and they do exist in Auth0 (either from being created via another tenant, else from being created in that tenant and then deactivated), and do manage to get a reset password email, then following the link will return the screen below:
System Administrator of the corresponding tenant should check if:
a) the user exists on that tenant, AND
b) the user status is active (listed under Users > Current).