This is not what happens in the example project so that would be something that you did, Pizza Tower did (search for gameframe_alpha), or decompiler bug if you are using a decompiled version of Pizza Tower without replacing the decompiled extension scripts by the actual ones from itch.
Yeah this is kinda a Pizza Tower mod that me and a friend decompiled with gameframe in it (im not pirating, basically every fangame and mod of pizza tower basically use gameframe) so this might look confusing but if you see when the caption (not the border) fades the border is still visibile, the only thing ik is they might use an older vers that i might prob not get in my hands or just straight modify the entire extension