Font size in title bar button - there only sometimes

I’ve got an app where the request is to show the font re-size option on all screens. This app uses a Content menu and the option is there for the menu as shown here.

However, if I enter any book collection, in this case the first one is the Pentateuch, and opening that to GEN or any other book removes the font size option from the top. Pinch and Zoom doe work but the request is to have the AA font button there. Any advice on how to make this show up for every book would be appreciated.