Currently there is no easy way of selecting the right build to copy artifacts from if the project we want to copy from is part of MultiJob.
The "Last Successful Build" is prone for selecting the wrong build if the project was build again in the meantime between the MultiJob start and current moment
and "Upstream Project That Triggered this Build" does not work as previously executed builds in the same MultiJob are not considered "upstream" (are not listed as "causes").
A patch to TriggeredBuildSelector has been created to explicitly support MultiJob plugin pipelines and query it for builds already completed, treating them as if they were "causes". Somewhat dirty (for in for loop ), but works correctly and for the number of projects in a pipeline does not matter much.
See attachment and a corresponding pull request.