A workflow-multibranch-plugin job configured with a branch source of a Mercurial repository with the above changeset graph – in which there is a single branch with two heads – results in a job for that branch which builds changeset 1. However, changeset 2, not changeset 1, is the tip of the branch. By extension, this means that pushes on top of the branch-tip are never built (although they do trigger builds).
This behaviour is present with workflow-multibranch-plugin 2.9.2 and 2.10-beta1, and with mercurial-plugin 1.57 and 1.58-beta1. I have not tested historical versions.