PS5 of https://gerrit.wikimedia.org/r/c/mediawiki/extensions/wikihiero/+/1293182 was marked as having a merge conflict before I rebased it through the gerrit UI. At https://gerrit.wikimedia.org/r/q/project:mediawiki/extensions/wikihiero+is:merged, you can see that only the two parent patches were merged recently, but nothing else. According to the commit message, the patch was rebased on a53c4998 when it was marked as having conflicts, which was the latest version of the parent patch before it was merged into master, so it doesn't make a lot of sense for the patch to have conflicts.
The same happened in https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1284048 (I broke the relation chain when rebasing, but it used to have a parent) and https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1293784, both of which had to be rebased through the gerrit UI and resubmitted because they were marked as having conflicts immediately after their parents were merged.
I feel like there must be a regression in Gerrit's merge conflict detection, or some other bug, as I've never seen this happen before until recently.