If you have a WDL that imports other WDLs to run, but the top level WDL has no tasks/calls of its own, then the API endpoint for getting submission outputs fails.
Response:
502: "message": "Object is missing required member 'calls'"
You can still make an API call to get the call-level metadata (above URL, minus '/outputs' on the end) and that works. But just getting the outputs via the API fails for this kind of submission.