In reference to the clarification gained in https://community.atlassian.com/t5/Bitbucket-discussions/Faster-feedback-on-merges-with-pull-request-pipelines/m-p/968332#M1494 I would love support for re-triggering pull request builds when the target branch changes so that the PR pipeline result is always on par w/ the merge product the PR would result in.
According to Aneita Yang this is not yet possible and next step regarding this would be creating a feature request, which I'm doing here right now. As it's my first one, please excuse if I pressed some wrong buttons or selected inappropriate values. Also please ask any additional questions.
For completeness I also attached that discussion thread from the reference as PDF.