When detaching nodes which are in a split-statement, the join line is still drawn for nodes on the left side.
For nodes on the right hand side it works fine.
See
http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuG8pk8eBSZ9Bk1I0m2hHq1hCI4ajIKpEWCWe9AOdPkP1vOq8o1jRSwNcfIY09K0iBWSW0RGA0000
code:
@startuml
start
split
:a1;
detach
split again
:a2;
detach
split again
:a3;
end split
end
@enduml
Tested with latest beta.
Regards