Upgrading from 1.24 to 1.26 version of the Multijob Plugin caused unpleasant side effect showed below, i.e. disappeared Parameters.
The fun fact that data about parameters still in the builds.xml file on Jenkins and in the same format as before (checked for diffs). But on attempt to manually type URL in browser's omnibox, Jenkins raises unobtrusive 404 error in debugger and redirects to the build details page:
I also would like to mention, that this bug annoying enough to force me rollback to 1.24 version, since page with parameters is heavily used by our devs.