Problem definition

When a plan branch is created, the build number for that branch starts at zero. This leads to very confusing situations when the build (or even a release) is not always triggered from the same branch. Functional workarounds exist, but since Bamboo doesn't reflect them in the dashboard, this causes confusion.

Suggested solution

Have a configurable plan option: "Ensure unique build number accross plan branches". This should make the buildNumber variable exist only once in the plan (and not for each plan branch). Every build within the plan (regardless from which plan branch) will then have a unique build number.
The minor downside of this approach could be that the build numbers for a particular branch are not sequential (however always ordered) and may contain 'gaps'.

Workaround

Currently, a possible workaround is to use a custom Plan variable as the build number. Use the "Version Variable Incrementer"-task from the Variable version tasks addon to increment that variable on each build. Though this works for having unique build numbers, it's causes confusion because the actual build numbers don't match those shown in Bamboo.