Documentation:
https://developer.atlassian.com/stash/docs/latest/how-tos/updating-build-status-for-commits.html

TeamCity plugin:
https://github.com/mendhak/teamcity-stash

I need to test Merge Result of Pull Request, so TeamCity monitor references refs/pull-requests/*/merge in Git and run builds on this commits. After commit TeamCity send Build Status into Stash.

Now Stash commit for reference refs/pull-requests/*/merge is not a part of Pull Request. But in other hand Stash look into this commit to say that Pull Request can't be merged if conflicts. A little bit misconfiguration.

In fact TeamCity can't not send to Stash 'previous' commit from refs/pull-requests/*/merge, because it's not guarantied that the build ran on the Automerge commit is also fine on previous commit. It's two totally different commits. So, TeamCity plugin makes the right call for valid commits. But Stash must attach this commit Build Status into Pull Request also as usual commit contains in Pull Request.
See discussion here:
https://github.com/mendhak/teamcity-stash/issues/2