I see a lot of people saying that it isn't so much an issue that a minority may be playing this role, but instead that white actors were excluded.
Could the reason for this be to improve readability between races and geographic areas? I'm not overly familiar with Witcher lore, but fantasy stories typically ascribe physical differences to the different races to create visual diversity and allow for greater readability (meaning, the viewer is less likely to start mixing up races and lore). If a story is to have several different groups, it can start to get confusing if all of them are represented the same way physically. Utilizing natural divisions between people is one of the simplest and most intuitive ways of doing this.