The maven release plugin assumes you are releasing from the latest commit on the current branch (HEAD). You cannot release from a previous build, it has to be the latest one.
When getting to the latest build of a module, the screen should be something like:
The default release version is defined in a variable named maven.release.version. The default next snapshot ( maven.development.version) is empty, and calculated by the maven release plugin.
If you want to know what will be the next release (or if you want to change it), check the plan variables configuration:
When you click the 'play' button to kick the release, Bamboo will allow you to customise a few variables. 
If this module is part of the reference application, other option you might want to override is the global variable 'preparing.refapp.distro.release':
When it's true, the refapp distro will be updated with the released version; when it's false, with the next snapshot.
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