If you use behat3, then make sure that its version has junit formatter support, as it was missed in early versions https://github.com/Behat/Behat/pull/676
Also check that you have configured profile at behat.yml
jenkins: formatter: name: pretty,junit parameters: output_path: ,build/logs/behat
jenkins: formatters: junit: [build/log/behat]
Also build.xml must call it as "behat --profile jenkins"