When you try to recall an email that has been routed via the Signature 365 service to add a signature, you receive a Recall notice informing you that the email failed to recall successfully, despite the email being unread in the recipient's mailbox.
Microsoft has confirmed that emails routed via connectors may fail to recall and that this is by design. The email property, which is used to find the email, is modified during the routing process.
This problem is not unique to Signature 365 but affects any email that routes via connectors.
There is unfortunately no workaround at this time.