When a main activity file includes two other activity files, in which first one also includes second one, then main activity diagram doesn't connect with the second one, letting line without connection:
@startuml
'https://plantuml.com/activity-diagram-beta
|Swimline1|
start
:preStep;
if (condition?) then (yes)
!include activity1.puml
else (no)
!include activity2.puml
endif
@enduml