This is intended behavior.
To design an "asynchronous" process triggered by another process, create it as a webservice without a response profile:
1. Replace the Process Call shape in the main process with either an http client connector or a web service soap client connector. This connector can be configured to call a process that has a web service server start shape.
2. Reconfigure the "sub-process" to have a web service server as the start shape to receive the invoking request. In the start shape web service server operation, do not specify a response profile, return data should be None.
By taking this approach, the webservice call should return with a HTTP 200 immediately and allow the process to execute independently from the parent process, thereby running asynchronously
The alternate approach Is to replace the Process Call shape with a Disk Connector or FTP connector to write data to a location where the second process(es) run on a frequent schedule to pick up the files and process them.