When we are trying to view the details of a process that has been called as a synchronous step from within another process, we are not able to view the sub process even after clicking on the + button.
Could you please let us know if anything has changed in this regard?
When you filter for a parent process name and then you see your instance after you click on the + sign that same filter on the process name is applied when attempting to view subprocess and so nothing is displayed because subprocess has different name.
Now if you filter for let's say primary object and then click on the +sign you will see your subprocess.