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I originally noticed this in my env. But after checking on the plantuml server, it looks that it does already gets inherited:
http://www.plantuml.com/plantuml/uml/7Ox1hOD030Nlhmet00LyVI5IG4wfmDbrY1Nmmguv8NgFoSqpiaPlRCxLzsK6saTjvSeBFMJRnPBSfJMUv3RhP9IrId8j1gIohCN4d9Yu4GDMNAZgz7Oghn37WTGDG_wZAXizaRkWT55R0_k-H_eF5mzgBfL_PKh5N6sNZFE4MAQh2unnniGl
Maybe you can check your version on your environment ?
@startuml version @enduml
Thanks