JENKINS-4972, the mercurial plugin discards merges for the purpose of triggering builds and determining culprits.
This makes sense when the merge is within a single named branch (i.e. it is during the regular flow of development).
It does not make a lot of sense to drop the merge when it is a merge across named branches. These merges are typically merges of a fix from one version branch to another version branch, and if this merge introduces errors, the merger wants to know about it.