After upgrading jenkins from 1.640 -> 1.642 and upgrading the plugin from 1.4 -> 1.5, the BUILD_USER_ID was not being set.
I reverted first the plugin then jenkins with no impact.
There was a warning in the Jenkins mgmt page that stated that this plugin had been disabled due to a stack overflow.
Warning no longer present, but the BUILD_USER_ID is not being set.
I can compare build variable dumps and it is absent in all recent builds where it was present in all builds prior to this last upgrade set.