"mandatory-script-verify-flag-failed (signature must be zero for failed check(multi)sig operation)"
After some additional research and helpful information found here, I was able to conclude the following:

Transactions encoded on 0.12.X bitcoin and then broadcast on 0.15 bitcoin may encounter "mandatory-script-verify-flag-failed (signature must be zero for failed check(multi)sig operation)"

This problem can be solved by doing the following:
Signing your 0.12.X raw transaction with a 0.15 signature. I cannot pinpoint the exact issue, however I was able to encode the raw transaction on 0.12.X and the sign it on 0.15 and successfully broadcast it. This information may be useful if someone is trying to figure out what parts of their signing process need to be refactored / updated.