I don't think it is due to the influence from Mandarin.
Maybe it is due to habit or dialect. For example, 罵 mē is sometimes realised as mēⁿ; vice versa, 麵 is mī but realised as mīⁿ yet some pronounced as mī.
Additionally, Mandarin does have voiced consonants such as [ʐ], [j], [w], [ɥ] and voiced nasals. What it does not feature are voiced plosives.