He was affected by a lack of consistent partnership, notably the Gini situation.
He had his best season in 17/18 and 18/19 when Gini and Robertson was playing triangles with him but last season when Fab was asked to become a defender, Henderson and Gini was also shuffled around and Mane ended up with some times Curtis Jones, Keita or Thiago.
While Salah has a constant group on his right side with Trent and Henderson, Mane had to do with a injury prone Keita and for last couple of matches Thiago that plays deeper and more centrally. This is very different to Gini and how Mane likes it.
Until Curtis comes back to help him out on the left, he might never rediscover his old form again.