Sometimes, when a series starts out with something impactful, it greatly increases audience's expectations.
But it is not particularly easy to keep that up continuously, making the rest seem worse than it is.

"Oshi no Ko" had that kind of effect, with the first double episode having that kind of impact, making the few subsequent episodes feel lacklustre before again having some great ones. But the "lacklustre" ones were still normally interesting episodes.

Similarly for "Mahouka Koukou no Rettousei", which with its well placed BGM (although now that I get back to it, I kinda don't like the tracks they selected) matching the scenes and great OP/ED complementing the story and the imagery making me feel a depth to the world, drove me to read the novel. And even though the subsequent seasons were extremely fun and even better in some ways, I didn't end up getting the same feeling.

"One Punch Man" seems to be having a similar problem.