Specifying link direction does not work between groups

Question

[A] -- [B]

  --> vertical: http://www.plantuml.com/plantuml/img/YtQCLT3LLOXsYWK0

[A] -r- [B]

    --> horizontal: http://www.plantuml.com/plantuml/img/YtQCLT0jqbM8Tee50000

Frame F0 {
  [A]
}
Frame F1 {
[B]
}
[A] -- [B]

  --> vertical: http://www.plantuml.com/plantuml/img/SojAp4rLS3DGgEPIK8XsZEMgvU9oWuWP0iMYdS0YG0a5NLq580y0

Frame F0 {
  [A]
}
Frame F1 {
[B]
}
[A] -r- [B]

  --> still vertical: http://www.plantuml.com/plantuml/img/SojAp4rLS3DGgEPIK8XsZEMgvU9oWuWP0iMYdS0YG0a5tI9T1I0N0000

 

I expected the last example would generate a horizontal link between A and B.

Thanks,

Mark