We have startet some processes which are running synchronously up to a task inside a subprocess. If we now try to find the main processes via the history API based on their processDefinitionKey, we get an empty result.

We do use the ReST-API for this purpose:
../history/process-instance?processDefinitionKey=...

Other process defintions that stop inside the main process can be found via the history api. The problematic processes can also be found via a runtime query