I may not be fully understanding your issue but one of the common mistakes I see when folks work with Service Request is that the field which stores the "REQ####" is the "Request Number" field not the standard C1 field most are used to. So your subject line should contain #RequestNumber# and email body should be pointing to that as well instead of the default of Change.
Because I really am not understanding what your issue is, and nobody else has responded, maybe you can look at the following:
PULSE article: Trending in Support: Email-Based Approvals
The issue is with the Response of Approval Mail which the approver sents.
If you will see both the above SS of mine you can see the Request ID not getting generated for one of the Notification Response and while for the other the Request ID is getting generated. Please find the SS of not working Response below
In the Request ID the Details Sig-ID should be passed from AP:Detail-Signature Form. Is there any Configuration that I'm missing.
(Note:-Using the same Email Template as used in the OTB Approval Mail and it is working for OTB Approval mail)
Thanks for your Reply.