Stagnation is not harsh and the level he reached is above what a lot of players can only dream.I get what your saying, but I think stagnation is a bit harsh. He’s been out with injuries and covid (from what I remember) and it’s been a very stop start season for him.
he’s young and will go through spells of being out of form. De Bruyne, Sterling, Bernardo have all been out of form for spells this season
last season really was his breakout year
Young players go through troughs and highs, he is not different, it's how he responds to the troughs that will determine how good he will become.