As of Bitbucket 7.4, with the introduction of the modern Core API for builds:

/rest/api/1.0/projects/<PROJECT>/repos/<REPO>/commits/<commitSHA>/builds

and request body representation example

{
    "key": "TEST-REP123",
    "state": "SUCCESSFUL",
    "url": "https://bamboo.url/browse/TEST-REP1-3",
    "buildNumber": "3",
    "description": "Unit test build",
    "duration": 1500000,
    "name": "Database Matrix Tests",
    "parent": "TEST-REP",
    "ref": "refs/pull-requests/xxx/from",
    "testResults": {
        "failed": 1,
        "skipped": 8,
        "successful": 0
   

the build result will not be displayed on the Pull Request builds tab for the above example where the build is for refs/pull-requests/xxx/from, even if the build was triggered, and the build server successfully sent the above payload to Bitbucket.

The page would only display a build for the source branch (for example for ref: "refs/heads/sourcebranch"

It would be useful to display the build result for the pull request refs, as the event and payload are received and stored in the database.