Steps to reproduce:
Create a deployment plan in bamboo specs on a branch that is not master
Create a linked repository in bamboo which points at that branch, and enable bamboo specs on it
Open the deployment page and edit it
Click "Use a custom plan branch"
Select the branch your specs are on
Click save
Click "Edit build plan again"

Expected behaviour:
The used for the deployment branch is whatever you set it to, or the option to change the branch is not accessible

Actual behaviour
The branch selection is cleared on reload

Workaround

Set the linked repository for the deployment plan as the one created above which had the branch the bamboo specs were on as it's main branch