This is a duplicate of bug 1545387, which has been resolved. See bug 1545387 comment 7 for more info on what happened but tl;dr: the try repo on the private hg service didn't have the hgrc file replicated, causing revisions to be pulled down as public which made the revset of candidate revisions for an artifact build to return the wrong changesets.
Bug 1548498 Comment 5 Edit History
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This is a duplicate of bug 1545387, which has been resolved. See bug 1545387 comment 7 for more info on what happened but tl;dr: the try repo on the private hg service didn't have the hgrc file replicated, causing revisions to be pulled down as public which made the revset of candidate revisions for an artifact build return the wrong changesets.