this manga sometime do it too, but somehow i don't hate it, i wonder why.. 😅
Maybe like you said, because when he shows up, it feels special.
Often, story with OP MC have to keep showing off and steal the show while insincerely “humbling” themselves.
Nakatani is kind of like Rei. They’re OP, but after doing their part, they quickly step back to the background and give the stage to the supporting characters (who usually got more colorful personality).
And while he does have the typical lack of confidence in earlier chapters, if we go with what Akane described now and previously, he already has a sense of purpose in this world and is doing his part in it. Rather than acting like he is not (which happens with both confident and unconfident OP MC).