We saw this too a few times - it seems this is a GitHub issue, that sometimes it fails to update the PR merge branch. This can only happen in case of GitHub PR builds as there we check out the GitHub provided /pr/merge branch, which should be the state of the code after it’d be merged.
Unfortunately it seems that GitHub sometimes in the last couple of weeks started to lag behind with the update of this merge branch…
A fix is already on our roadmap for this week, to do a full manual merge instead when possible, instead of relying on the github provided merge branch.