Is not a problem of my mod but I think I know what you mean. Go to your Tzardoms folder, mine is C:\Medieval II - Total War\mods\Fall_of_Constantinople_1.5 and open a file called "Fall_of_Constantinople_1.5.cfg" with a text editor like notepad.
Search for the line [video] and under it make sure you have it like this:
borderless_window = 1
windowed = 0
if not, just change it to that and save the text file with the changes
Teoclestzardoms
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