The signature that appears on your picture should match what's on certificate, but if your middle name is missing on picture signature it doesn't invalidate the certificate. If you had a totally different name on picture signature, then you may have had an issue, but all you are missing is middle name on picture signature. I guess you didn't sign the actual certificate since you sent it back?
What you're describing is a non issue, but since you already sent it back there's nothing you can do no but wait. It may take a few weeks to receive your corrected certificate. Some have signed picture signature with eligible signature..but it doesn't invalidate the certificate.