With a build pipeline where a downstream plugin is triggered manually from the pipeline view, the BUILD_ environment variables are set from the originating build at the start of the pipeline instead of from the user who manually triggered the build.
I suspect the build user vars plugin is recursing the getCauses or upstreamCauses until it finds a UserIdCause, however this only deals with the first causeAction of each build.
It should rather get all causes using: build.getActions(hudson.model.CauseAction.class)
and recurse the cause chain similarly by referring to: cause.upstreamRun.getActions(hudson.model.CauseAction.class) instead of cause.upStreamCauses
The triggering userId can be found as one of the causes of the build itself, rather than chasing up the upstreamCause
More detail here: https://stackoverflow.com/a/52816573/2332068