For now, Dean’s time as Michael’s vessel may actually be beneficial to the Winchesters and the other hunters.
He’d undoubtedly be a wreck if he had recalled everything as soon as Michael vacated his body.
Dean doesn’t remember those three weeks, which is really for the best.
While I’m guessing at least some of those memories will come back, at least this situation seems to be different from when Dean was concealing his memories of Hell way back in Season 4.