Hello B.,
- In which version of PlantUML ?
Because with the last version (v1.2020.15) your code works:
@startuml
<style>
wbsDiagram {
RoundCorner 3
}
</style>
ent1 ||--o{ ent2
@enduml
[Click to see on PlantUML online server]
or with a test of arrow color:
[Click...]
If that can help,
Regards,
Th.