The word bias has a negative connotation so perhaps you mean something else.
Given two EMs, one that came organically from a team and another that came "from the outside" and spent a few months working with a team - what advantages and disadvantages would the first EM have over the second?
IME internally promoted EMs are 4x better. They understand what problems the team is facing, how the product works, the various interactions with other teams, and the company processes. It takes at least a year for an external hire to ramp up to that level of familiarity.
I will say that for very dysfunctional teams it could be valuable to get an outside perspective which can do more severe change but hopefully this is less common.
Given two EMs, one that came organically from a team and another that came "from the outside" and spent a few months working with a team - what advantages and disadvantages would the first EM have over the second?