Pull request pipeline does not show link to PR after 2 reruns

Currently, if you have a Pipelines .yml file that triggers for PR, then when you re-run, stop, re-run, stop your build, the PR link in the build is not showing.

Steps to reproduce:

1. Copy the .yml file below.
2. Create a branch
3. Create a commit to that created branch
4. Create a PR pointing from your created branch to "master" branch (This will create a Pipelines build)
5. Go the generated Pipelines build
6. IMPORTANT Step: Re-run, Stop, Re-run then Stop the build.

Another use case with the same outcome:

Just adding here another case where this occurs:

1. Let's say first build on a PR is build 1.
2. Re-run build 1, so now you have build 2 (re-run of 1).
3. Every re-run of build 1 will maintain the link.
4. Every re-run of build 2 (which is already a re-run) will not maintain the link.

#!python

image: docker:stable
 steps:
    - step: &build
        name: Build, test and push image
        script:
          - echo "Script test"

pipelines:
  pull-requests:
    '**':
      - step: *build

Expected Result

The PR link will still be there after some series of re-runs

Actual Result

The PR link isn't showing after some series re-runs