During the ~last half year changes were made to the Bitbucket Branch Source - plugin regarding the Bitbucket Post Webhook - Pull Requests - events.

https://github.com/jenkinsci/bitbucket-branch-source-plugin/commit/e6d9e7a3721116d99bc24d89e7abee1bd53f6e9f

https://github.com/jenkinsci/bitbucket-branch-source-plugin/commit/cfaa954953aa5b84a0e5d9edad775a8301320278

From my understanding up to ~ summer 2025 the plugin did not define the pull requests - events explicit. Since then they are explicitly listed, but some have been missed or were not added by purpose.
Outcome is following: The initial created PR is automatically built by Jenkins. When additional commits are added the build is not triggered anymore. 

The root cause is die missing "Pull request re-scoped" event at the webhook. See attached screenshot.

I checked out the plugin-sources, added PULL_REQUEST_RESCOPED to 
com.cloudbees.jenkins.plugins.bitbucket.impl.webhook.plugin.PluginWebhookManager and so on. When i deploy this custom build of the Plugin everything works as expected again. I´ll try to provide a PR for this.