A have example:
<pre>
skinparam usecaseActorBackgroundColor white
skinparam usecaseBackgroundColor white
skinparam usecaseArrowColor black
skinparam usecaseBorderColor black
left to right direction
usecase 1
usecase 3
usecase 4
usecase 6
usecase 7
usecase 8
usecase 10
rectangle "Foo" {
rectangle 2
rectangle 5
rectangle 9
}
1 .. 2
2 -> 3
4 .. 5
5 -> 6
5 <-> 7
8 .. 9
9 -> 10
</pre>
With output:
Problem is with border color of nested rectangle.
PlantUML version is 8021.
Kind regards,
Michal