Mercurial subrepo (.hgsubstate) changes don't always show up in PR diff

Probably not always but frequently enough changes to existing subrepos of a Mercurial repository (which changes are recorded in the .hgsubstate file by Mercurial) don't show up in a PR of the parent repo, the PR is just empty. The change however is really there in the branch, it's really a diff to the target branch.