If the built branch is updated via a merge commit rather than a change, the build is triggered regardless of whether the files updated in the merge match the included regions regexp or not.

A test repo can be found at http://blank.org/memory/work/jenkins-bug/repo/jenkins-bug/regions-test.git. A config.xml is attached to this ticket. Note that commit 10c1279f32a5ffe9f6eabfe01aa360341144c263 in that repo (merge testbranch->master) triggers a build.