Process Property value within a sub-process is not available to parent process.
Process properties have a global scope and should be accessible to a parent process. However, a process property value set or modified in a child process can be retrieved later in the parent process after the child execution completes. The key here is the child execution has to complete, otherwise, process property value will not be available to parent process.
Check the "Wait for process to complete" checkbox in Process Call shape's General tab.