It’s correct that the collection variable doesn’t show the substituted values (mine is the same). The collection variable contains the literal value that you asked it to store; when the body is submitted, the collection variable is substituted into the body, and that’s when the dynamic variables are replaced.
It’s weird that your screenshotted response has a completely empty data value - this indicates that it’s substituting {{req_body}} with a blank value. Out of interest, do you have a req_body variable set up in your environment variables? Because of the way that variable scopes operate, if you had a variable with the same name in your environment variables then this would take precedence over the collection variable set by your script.