In context of migrating process instances to camunda BPM (see https://github.com/camunda/camunda-consulting/tree/master/snippets/migrate-third-party-to-camunda) I found two bugs in the engine:
1.) https://github.com/camunda/camunda-consulting/blob/master/snippets/migrate-third-party-to-camunda/src/main/java/org/camunda/bpm/engine/impl/pvm/runtime/AtomicOperationProcessStartInitial.java#L56: The activity of the scope has to be checked. And if this is done even recursivly (as in the code here) we can jump into sub-subprocesses as well (works in migration).
2.) https://github.com/camunda/camunda-consulting/blob/master/snippets/migrate-third-party-to-camunda/src/main/java/org/camunda/bpm/engine/impl/persistence/entity/ProcessDefinitionEntity.java#L89 - the list of executions should not be resetted!
Both fixes should be applied to the core engine and merged back to 7.1.5 - I will trigger the customer to create a corresponding support ticket.