The "Working Copy" tab is a project-level link, and always goes to the working copy of the most recent build. From a Build Result page, it may lead to a different working copy from that which generated the current Build Result.
For example, a project may be scheduled to build on various hosts (and architectures), each with its own working copy. Nevertheless, the "Working Copy" tab on every Build Result page in its history goes to the working copy of the latest build. For an Ant or Shell script build which does not copy its build artifacts to a central repository, this means that you can get the build artifacts from the working copy of only one architecture at any given time.
If users need access to a project's working copies for distributed builds on multiple hosts, then you should provide that access by other means.