Changing the target branch of a pull request clears the lastReviewedCommit column

Issue Summary

If on an existing pull request the target branch is changed after reviewers have completed their reviews the lastReviewedCommit column in the sta_pr_participant table in the database is NULL -ed but review verdicts (Approved, Needs Work) are maintained.

Note:
The above is only unexpected if the original and the new target branch are at the same commit. If they are not at the same commit, then Bitbucket Server correctly removes the review verdicts and correctly clears the lastReviewedCommit column.

Steps to Reproduce

1. In a repo, create two branches off master, branch1 and branch2.
2. Make changes and push to branch2.
3. Create a PR from branch2 to master and add a reviewer
4. As the reviewer approve the PR. At this point the lastReviewedCommit column in the sta_pr_participant table will be set
5. As the original user, update the PR and change the target branch from master to branch1.

Expected Results

PR approval is kept and lastReviewedCommit still has the same value as before

Actual Results

The approval of the PR is kept (this is expected because the original and target branch are at the same commit), but the value of the lastReviewedCommit column in the sta_pr_participant table is now NULL.

Workaround

There is no known workaround.