The answer is as simple as the predictable sophomore slump, a disease which afflicts almost every professional athlete in almost every sport. Mainoo was outstanding last season and there is an elite footballer there, but he's well off what we saw from him last season. Whether he can ever return to his form last season is an open question, but those who question whether his talents fit in Amorim's system may be onto something. Maybe, as pretty much every player in the squad bar Amad and until the last few games Onana and Mazraoui has been disappointing this season for whatever reason.
When shit grows around you, sometimes it grows on you.
In any event, there was zero chance Mainoo was ever going to maintain the kind of form in his second season that we saw in his first season. Keep playing him, rotating when necessary, and take it easy when his performances are nothing more than competent but far from outstanding.