Bamboo should not reuse Plan Keys from deleted branches

Steps to reproduce:
1. Manually create plan branches and bamboo will automatically assign a Plan Key to them. eg: FOO-0, FOO-1, FOO-3.

2. Create multiple deployment versions of FOO-1. Say the last one wast #40, which is release FOO-1-40.
2. Delete FOO-1 plan branch.
3. Keep FOO-2 and FOO-3
4. Create a new branch plan.
5. Sometimes Bamboo will reuse and assign the FOO-1 plan key to the newly create branch. that is when I have problems.

6. Try to create a release from the FOO-1, say release FOO-1-${buildNumber}, which translates to FOO-1-1.

Bamboo will complain that FOO-1-1 cannot be create because it is lower than FOO-1-40 (from step #2 above).

Also, in the create release page, when I select the branch behind the FOO-1 plan key, it will list the build results created by the branch that was previously pointing to FOO-1.
That is how I came to realise bamboo was reusing old plan keys from deleted branches.

I hope my explanation isn't too confusing.