Hello C.,
From:
@startuml
rectangle A
rectangle B
rectangle C
A ------> B
A ---right---> C
@enduml
A possible workaround, for long arrows, is to use norank, as:
@startuml
rectangle A
rectangle B
rectangle C
A ------> B
A -[norank]-------> C
@enduml
See also:
If that can help,
Regards,
Th.