A pull request whose branch was pushed on top of another's sometimes gets no CI and reports conflicts that git does not have

Seen on 2026-10-22 with #362 (branch 361-home-tab, branched from #360's branch, which then merged). The PR showed merge_base null. Pushes at 04:43 and 05:15 created no CI / pull_request run at all; the required CI sat at 'expected'. update_branch answered 'conflicting' on home.tsx, ground-truth.tsx and home.md, though main's only new commit was #360's merge commit, whose content the branch already had (git merge origin/main was clean). A local merge of main pushed at 05:26 got CI at once. #364, branched from main, got CI within seconds. Find why the pull_request trigger and the branch update misjudge the merge base for a branch stacked on a merged PR.