It looks like the appveyor commit SHA needs similar fixing as the Github Actions or Azure one.
In https://ci.appveyor.com/project/Flamefire/nowide-fr98b/builds/38560544/job/kb2fl8uw27ued5qs the commit is detected as b6c2ea6c45b76b760189abe5246df472e701a1f9 while it should be 0c853652e5409b32ef8d07939d93c4c5d7e55094
See the same CI on GHA: https://github.com/boostorg/nowide/pull/131/checks?check_run_id=2272049938
The correct env var for PRs seems to be APPVEYOR_PULL_REQUEST_HEAD_COMMIT