Stage gets auto completed when first child task completes

Steps to produce:

1. create CMMN model
2. the CMMN should contain a stage with at least two child process tasks
3. the stage is configured with autoComplete = true
4. deploy the CMMN model
5. create a new instance

Expected Behavior:
All child process tasks have been executed

Observed Behavior:
When the first process tasks complete, the stage gets completed (because there are no active child tasks)