Problem Definition

It can be confusing that the rest/api/latest/result/$PLAN_KEY/branch/$branchName API does not work for the default branch (often master).

Suggested Solution

Allow the branch API to also lookup the default plan (and thus default branch)

Workaround

If you want to retrieve results for the default plan (and hence default branch) you should instead use rest/api/latest/result/$PLAN_KEY. By not specifying a branch, you are implying the default plan.