The parserRadioButtons.pl file has been substantially rewritten for v2.11, and the changes should resolve the issue you are having.
See the pull request for more details. Unfortunately, you can't just upgrade the parserRadioButtons.pl file, as there are other changes that are needed to support the new version (see the "changed files" tab in the pull request lined above). You would have to upgrade to the develop branch (or the release-2.11 branch) to get access to the new code.
WeBWorK Problems
Problems with NAMED_ANS_RADIO in PGbasicmacros.pl
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