Hello H., C.,
Another workaround, you can use sub-diagram as:
@startuml
skinparam FrameBackgroundcolor white
frame userA [
{{
card root
card sub1
}}
]
frame userB [
{{
card rootB
card subB1
}}
]
userA -up-> userB
@enduml
But with that we will lost tab name and the sub-element are no longer linkable.
See also this answer: https://forum.plantuml.net/9875/arrow-is-starting-on-a-strange-side-of-file?show=11586#a11586
If that can help,
Regards,
Th.