Well it looks like this may be a known issue, which was fixed in SEP 14 MP1, pulled from the fix notes:
Reboot settings do not appear to work after installation
FIX ID: 4038133
Symptom: You create an installation package with custom settings, a silent installation with a delayed restart. After installation, however, the restart prompt does not match the delayed restart. If the client you install does not include the firewall, the post-install reboot request sometimes does not follow policy.
Solution: When Symantec Endpoint Protection is installed without the Firewall feature, Symantec Endpoint Protection no longer requests a reboot without honoring the installer reboot policy.
Probably best to get a supprot case open.