Try changing the formal title in File ▸ Compile… under the metadata tab on the right hand side of the overview screen. When you created the project that will have been set to the name of the file you saved to the disk, but it would not presume to change the title if you modify the name on the disk at a later time (we might use file names for something entirely unrelated to the title of the book, like date stamping them, or marking who has last proofed it), nor does it even know what a “title page” is—that’s just a file in your binder like all the rest.
After changing it, close the project to update the icon and Quick Look preview.
It sounds like you are thinking of that text file in the folder as being something special, like a feature, but it’s just a file in a folder with some text typed into it. The way you worded that even demonstrates how confusing this would be for a machine to figure out—you have title pages, plural. What happens if one is different? And then even if they are all the same, how does mere software figure out which part of the title page is actually the title—not the date, or the copyright, or your name?
The actual feature is the one in the compile interface. That is what you should use to change the title of the book, and that is the only thing that the software can reliably “know” to be your title.
Thus a better solution would be to make it so your title page uses the real document title, rather than typing it in by hand, separately for each text file where it appears. Try the <$projectittle> placeholder—or take a look at one of our built-in templates and see how they put together title pages in a flexible manner.