I found this which suggests 129 is the SAP exit code for a restart being required.
https://www.reddit.com/r/SCCM/comments/r522ek/sap_gui_install_via_psadt/
If that is the case then maybe you could add actionscript so if exit code 129, it will exit with code 3010 which may allow the Bigfix action to see it as Pending Restart, or of course you could force a Pending Restart by using action requires restart.