I’ve been thinking that as well. That it has more to do with her being a “woke" foreigner than anything. She came into a very old fashioned, centuries old establishment with a lot of new age ideas and believes. They were bound to clash.
Diana and Wallace were both treated badly. Mostly for being women with view points that didn’t align with the old ways. Although it has been said Wallace was not a very nice women. Could have been some of that.
I think it goes more to sexism. The public in general can’t get behind a strong women with strong opinions. Regardless of their race or nationality .
This explains why The Queen and Markle seem to get along. It’s that common ground of being driven women in a man’s world.