I've just done a test here with both Ubuntu 14.04 and Ubuntu 15.04 versions of util-linux (2.20.1-5.1ubuntu20.4 vs. 2.25.2-4ubuntu2), and I can confirm that the older version of lscpu gives different output on the same kernel. So yes, this is a bug in util-linux's parsing of /sys.
I've just done a test here with both Ubuntu 14.04 and Ubuntu 15.04 versions of util-linux (2.20.1- 5.1ubuntu20. 4 vs. 2.25.2-4ubuntu2), and I can confirm that the older version of lscpu gives different output on the same kernel. So yes, this is a bug in util-linux's parsing of /sys.