Can't filter on Bitbucket PR source Branches instead of Target branches

[Suman Das]

I have a requirement where i need to filter branches for a build based on the source branch of PR.
Currently build is triggered anyways and have to write custom logic inside the jenkins script to handle the further build steps prevention on that branch as my filtering criteria needs to be the source branch and not destination branch
Is there a reason why we use only destination branch for the plugin branch regex matches (for PRs) and not source branch ?