Nagi no Asukara ending was pretty mediocre. Until then it was great.
TQQ fell a bit off at the ending stage due to a certain lack of development on the winning quint and MC's interactions.
I feel like Darling in the Franxx made Zero Two a bit of reverse development in the latter half undermining the quality of the anime in that half.
Dragon Ball GT/Super. Not same quality as Z and OG.
Sound! Euphonium movie sequels were not as good as S2 but we'll see in S3.
Magi changed from a dungeon adventure to an higher than thou philosophical ideology battles at the end. Underwhelming
Mirai Nikki skipped manga scenes and it fell in quality in the later eps.
Akame ga kill manga ending > Anime ending
^ Same for Soul Eater and Shaman King
Zero no Tsukaima S1 and S4 > S2 and S3