Check if rpmbuild return value is 0 when failing (echo the command return value to see), if so this is normal and rpmbuild is to blame for this else it's Jenkins fault.
If it fail and still return 0 you will have to parse the console output and check for the failures strings using Console Log Parser plugin or something like it. Else you can do the same but just mark the check as temporary until the appropriate channel fix it.
Are you using something that wrap the rpmbuild command?
Jerome