Would you say Alisson is "solid and consistent" by way of skill definitions? I wouldn't. And you speak about the two derbies, isn't it true that De Gea didn't save a shot in the first 3 or so matches in the WC?
What about Courtois? Currently getting hammered in Madrid by fans who want him dropped.
Kepa is still relatively unproven in the PL. In City's biggest game last season away to Liverpool in the CL, with the game finely balanced Ederson passed the ball to Salah (I think) who lobbed him.
This is the necessary context for judging our GK's ability / form.
Was Sunday's mistake awful? Yes.
Should be be criticised for the mistake? Yes.
Should it be used, together with the other Derby mistake, as a barometer of his ability? Absolutely not, given what I've typed above and his performances so far this season (largely very good).