Hello G.,
It seems a bug.
A possible workaround is to use style as:
@startuml
<style>
spot {
FontColor White
FontSize 16
}
</style>
skinparam CircledCharacter {
FontColor White
FontSize 16
Radius 20
}
class "Class 1" as class1 <<C,red>> {
}
@enduml

@PlantUML team:
- What is the radius param. on style?
Regards.