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