MinimumWidth doesnt work when there's a nested object

Question

If I use the example below, rectangle A follows the MinimumWidth constraint as expected, while rectangle B adjusts its width to only fit the nested elements. Is this normal behavior or a bug? And how can I make sure both rectangles have the same width without adding extra spaces or components?

@startuml
<style>
rectangle {
  MinimumWidth 400
}
</style>

rectangle A {
}

rectangle B {
  card Hi
}
@enduml