Can't merge a stacked PR with no clear reason (all PRs show mergeable)

[mr04vv]

Environment

• develop branch are protected by a repository ruleset with:
  • require_last_push_approval: true
  • dismiss_stale_reviews_on_push: true
  • required status checks
  • No merge queue.

Reproduction

1. Create a 3-level stack: develop <- A <- B <- C.
2. Approve every PR after its latest push. All three show MERGEABLE / CLEAN / APPROVED, all checks green.
3. On the top PR (C), the native GitHub "Merge stack (3)" box shows "Able to merge as a stack". Click it (merge method: Create
   a merge commit) and confirm.

Expected

All three PRs merge into develop, or the operation stops before anything merges.

Actual

• UI shows "Merging stack… / 3 pull requests are being merged" briefly, then rolls back to "Able to merge as a stack". Nothing
  merges.
• The PR event log records repeated auto-merge was automatically disabled — "Pull Request is not mergeable".

（In the image, the bottom PR has been merged, but the behavior remains the same even if it is not merged.）

Notes

• Root cause not confirmed — the above are observed events only. One hypothesis is that merging the bottom PR moves the
  merge-base of the PR above it, invalidating its latest-push approval, but this has not been verified.
• Reproduces without a merge queue, which differs from Stack merge hangs after partial merge invalidates latest-push approval #174 (scoped to merge-queue branches, describes a hang rather than a
  rollback). Filing separately because the trigger and symptom differ; feel free to consolidate into Stack merge hangs after partial merge invalidates latest-push approval #174 if preferred.

[shanegrouber]

Same class of failure on a 3-PR stack without a merge queue (repo: private product stack, trunk master).

Setup

• Stack: master <- #A <- #B <- #C (all open, squash intended)
• Every PR: mergeStateStatus=CLEAN, mergeable=MERGEABLE, reviewDecision=APPROVED, required checks green
• No merge queue on the repo
• Clicked Merge stack from the tip (#C)

Actual

• UI stuck on “Merging stack… / 3 pull requests are being merged” with all three rows showing Merging
• API after several minutes:
  • #C (tip): autoMergeRequest enabled (squash), baseRefName still the middle branch (not trunk)
  • #A and #B: still OPEN, autoMergeRequest: null, bases unchanged
  • Trunk tip unchanged — nothing merged
• So the stack job appears to arm auto-merge only on the tip. Tip cannot merge until lower PRs land and bases retarget; lower PRs were never started → deadlock (infinite “Merging…” rather than a clean rollback)

Contrast with this issue’s “rollback” path

In your report the UI returns to “Able to merge as a stack” and logs auto-merge disabled / not mergeable. We stayed in the merging state with only tip AM on. Same broken bottom-up orchestration; different UI terminal state.

Workarounds that work

• Cancel stack merge → merge bottom-up (or enable auto-merge on #A first), accepting per-layer CI after each squash
• Or retarget tip to trunk and merge tip only (full stack in one squash), close lower PRs as superseded

Happy to pull more GraphQL/API snapshots if useful.