I've just pinpointed this to v2.2.4 of the bitbucket-branch-source-plugin. In v2.2.3, {{CHANGE_BRANCH}} is still set according to expectation. Looking through the diffs, it seems to be caused by the changes on context of [the following pull request|https://github.com/jenkinsci/bitbucket-branch-source-plugin/pull/63] (for JENKINS-45775).
I've just pinpointed this to v2.2.4 of the bitbucket-branch-source-plugin. In v2.2.3, {{CHANGE_BRANCH}} is still set according to expectation. Looking through the diffs, it seems to be caused by the changes on context of [the following pull request|https://github.com/jenkinsci/bitbucket-branch-source-plugin/pull/63] (for JENKINS-45775).
Another problem this issue is causing happens when you open the pull requests tab, you don't see the name of the branch anymore, you only see the PR number:
In blue ocean you see the PR name which is nice, but it would be nicer to also clearly see the branch name of the PR.
Any update on that still existing blocker issue? Rein Achten seems to have done some useful reasoning work in his comment. Does it seem reasonable to you Stephen Connolly?