Schemes: target/build type map doesn't seem to support non-array strings

I had a scheme like so:

schemes:
  MyApp:
    build:
      targets:
        MyApp: all
        MyAppTests: test

However, the generated Xcode project showed it (in Xcode) with all build types enabled for both targets.

The docs seem to say this syntax should work: targets: [String:String] or [String:[String]]. (I'm assuming the above code is the first of those syntaxes.)

However, I had to change the YAML to be an array for Xcode to do the right thing:

schemes:
  MyApp:
    build:
      targets:
        MyApp: all
        MyAppTests: [test]

Start with the build section in Docs/ProjectSpec.md and verify the documented [String:String] syntax against the generated Xcode project. Trace the scheme target configuration entry point, then reproduce the example with scalar and array values and confirm each target receives only its requested build type.