When source branch of Pull Request is updated then event generated by Bitbucket isn't associated with Pull Request. So it makes impossible to filter by PR branches. On the other hand when branch is passed to Jenkins then plugin checks association with PR and passes extra ref to Jenkins.
Currently plugin detects 2 types of pull request events
real event (create/edit/re-open/merge)
source branch change
This is a RefChange event and we do extra checks for PR in this case
Based on that, the plugin sets branches parameter that is filtered again on Jenkins side to decide if the job should be triggered. It requires extra config on Jenkins side and causes support requests which take a lot of time. (Like MS-3896)
So I would suggest to add "Trigger Jenkins for pull requests only" checkbox which will do all filtering on Bitbucket side (based on rules above) and require no extra configuration on Jenkins side.
The linked issue - has been resolved
Two questions to gauge priority:
How frequently is this an issue for customers? How many support requests does it drive?
How much time would the feature take to implement, including development, testing, and review?