The following works well
@startuml
''!pragma useVerticalIf on
start
if (condition) then (yes)
:Step 1;
detach
elseif (condition) then (yes)
partition "Partition" {
:Step 2;
detach
}
else (no)
:Step 3;
detach
endif
detach
@enduml
but if the pragma line is uncommented an extra decision box below "Step 3" is wrongly generated.