What happens if Cinderace V with the "Field Runner" Ability is in play while Galar Mine is also in play?
Cinderace V would have no Retreat Cost. When effects that increase something like Retreat Cost by a particular amount are stacked on top of effects that make something like Retreat Cost vanish, the vanishing effect takes priority.
Source: Rebel Clash FAQ (2020-04-18), TPCi Rules Team (2020-04-30)