Hello B., and all,
See similar reported issue here:
For your second cases, could you add an alias, as:
@startuml json "some json" as J { "a": 1 } @enduml
For the first case, @PlantUML team, see also this example:
@startuml json someJson { [1, 2, 3] } @enduml
If that can help, for debugging...Regards.Th.
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