Hello W.,
As mentioned on:
For a possible workaround, you can use:
skinparam SequenceMessageAlign first
Without, all is Left-justified:
@startuml
A -> B: Left-justified\ntext\ntext
A <- B: Left-justified\ntext\ntext
@enduml

[See on PlantUML server]
With, we observe Left-justified and Right-justified:
@startuml
skinparam SequenceMessageAlign first
A -> B: Left-justified\ntext\ntext
A <- B: Right-justified\ntext\ntext
@enduml

[See on PlantUML server]
If that can help,
Regards,
Th.