I am using Bitbucket server with Bamboo. I have a linked repository between the two and have enabled "create plan branch with pull request is created", but I don't see any plan branch created.
My setup:
Bamboo 7.1.2 linked with the following:

My bamboo setup:

And

I see in the logs that bitbucket pull-requests are received:
2020-10-21 17:11:28,736 INFO [http-nio-8085-exec-23] [AuthenticatorImpl] Authenticated app 'Bitbucket:5482248460' as user 'null' successfully
2020-10-21 17:11:28,738 INFO [AtlassianEvent::0-BAM::EVENTS:pool-1-thread-12] [PullRequestAwareEventListener] Received pull request opened event [ part-recognition, from: refs/heads/VCVCOR-841-gtest_discover_tests, to: refs/heads/master]
but no pull-request plan branch is created and no pull-request builds happen.