The reason is that the backend_rawhide_to_release command that I used for transition of builds from rawhide to f26 copies only the latest successful builds for each package. Your needed dependency, however, comes from a build 468985 that was marked as failed but still contains (as the only one) the required package kde-workspace-libs that in turn contains libtaskmanager.so.4. What was actually copied was the resubmitted build 469002 containing only kde-workspace-libs for f23 chroot that failed in the previous build. Smarter backend_rawhide_to_release command could be implemented that looks at individual chroots and copies them from even failed builds. For this case, I, however, just copied the build 468985 into fedora-26-x86_64 and fedora-26-i386 chroots (ppc64le was not built).