Answer
Grant EXECUTE on the custom package explicitly to IFSSYS: GRANT EXECUTE ON IFSAPP.<custom_package> TO IFSSYS;. Standard IFS packages already carry the grants IFSSYS needs (SELECT on views, EXECUTE on methods, and SELECT/UPDATE/INSERT on tables with LOB columns), but a custom package doesn't get that automatically — it has to be granted manually. If the error persists after granting to IFSSYS, also grant EXECUTE on the same package to IFSCONNECT, since that's the user the process is actually running as in this integration flow.