Example code. Render and check the arrow end. Shouldn't the arrow end into the boundary and not go inside the pointed object?
@startuml
rectangle A
rectangle B
rectangle C
rectangle D
rectangle E
rectangle F
A --> B: 1
A -[thickness=2]-> C: 2
A -[thickness=3]-> D: 3
A -[thickness=4]-> E: 4
A -[thickness=5]-> F: 5
@enduml
