Alright, that's a really unpopular opinion, but: Haikyuu was easily one of my favorite series in Jump, however, after the start of the nationals arc my opinion of the series declined considerably. It had great moments here and there, and the art is still great, but the games got way too dragged out for me, and there was way less of a cohesive throughline in them, just a collection of isolated moments, most of which didn't work. The other teams (with one exception) were also way less interesting. And the recent developments of the story didn't help it recover. I don't find it terrible or anything, but for a series I once liked so much I've been terribly uninvested for almost half its runtime now. There are other series I felt got worse as they went on, but none I liked so much before.
I can't really think of an example of the opposite. Maybe Chainsawman? But the period when I didn't like it very much is just like 10-15 chapters, and I still found it okay, just not great.