Hello K.,
Awaiting management by style (I will post wanted feature later);
From my first comment and using the global parameter (because defaultTextAlignment is a global parameter... not possible to use on stereotype):
skinparam defaultTextAlignment
and sub-diagram or embedded diagram, a possible workaround will be:
@startuml
skinparam defaultTextAlignment center
rectangle "RIGHT Group\rRIGHT RIGHT RIGHT RIGHT\rRIGHT RIGHT" as supR {
rectangle "LEFT Group\lLEFT LEFT LEFT LEFT LEFT\lLEFT LEFT" as supL {
rectangle supC [
<b>CENTER Group
<b>CENTER CENTER CENTER CENTER
<b>CENTER CENTER
{{
skinparam defaultTextAlignment right
rectangle "RIGHT Component\rRIGHT RIGHT RIGHT RIGHT\rRIGHT RIGHT" as subR
}}
{{
skinparam defaultTextAlignment left
rectangle "LEFT Component\lLEFT LEFT LEFT LEFT LEFT\lLEFT LEFT" as subL
}}
{{
skinparam defaultTextAlignment center
rectangle "CENTER Component\nCENTER CENTER CENTER CENTER\nCENTER CENTER" as subC
}}
]
}
}
@enduml
[See on PlantUML online server]
If that can help,
Regards,
Th.