Here’s an annotated version of your screenshot of the Project Info dialog - what you’ve input into the Subtitle field is what you should (could*) instead put into the Title field for the flow, which I’ve circled in yellow. That’s what I meant about “on the left” of this dialog.
You can have any number of flows in a single project - each flow can have its own title, with a different composer etc as well if necessary. The Project Info dialog is where you manage all this information: the first entry in the list on the left of the dialog is the project as a whole (e.g. “Symphony No.1”), and then below it is the separate flows (e.g. “Mvt I: Allegro”). When you click a different flow on the left, the information on the right updates to show you what’s set for the selected flow.
(* as Leo has said, you can instead leave the subtitle where it is, and just use the token {@projectsubtitle@} to show it wherever you want in a text frame. There are often different ways you can achieve the result you want. But if you want to use the automatic flow heading that gets shown at the top of the flow, I recommend using the Title field for the flow.)