I'm not sitting at my computer to check but this sounds like it might be a known limitation of the yum module listed as the last note in the docs: http://docs.ansible.com/ansible/yum_module.html#notes
Yum has two kinds of groups (package groups listed in the spec file when building the package and environment groups listed in a separate xml file when building the yum repository.) Sometimes yum will treat these two interchangably. other times it requires you to specify them using slightly different syntax: "@group" vs "@^group". Unfortunately, the interface ansible is using requires the separate syntax so you have to decide if you are dealing with an environment group or a package group and change your group name appropriately.
-Toshio