Hello! I'm also working with strictuml, but I didn't understand it as a bug report.
In UML 2.5.1, arrows in use case diagrams are open, except when using generalization (closed unfilled triangle). There are no closed-filled triangles in the notation. Isn't it so?
Those are the expected behaviors in a Use Case Diagram, as far as I'm concerned:
@startuml
skinparam style strictuml
(MyUseCase1) .> (MyUseCase2) : <<include>>
(MyUseCase3) .> (MyUseCase4) : <<extend>>
(MyUseCase5) -|> (MyUseCase6)
'The latter would be a generalization.
@enduml