My father was adopted, so I have a last name that if I took a DNA test, I’d very likely not match it at all. It isn’t that unusual. I usually say “the last name is Italian, I am not.” If they press on my ethnic origins I’m likely to say, in a not necessarily sweet fashion “why do you want to know?” I don’t find it that interesting, and I don’t think it adds anything to someone knowing me to know I’m 1/16 Dutch.
I don’t think its a microagression, I just think its - in the old fashioned term - an impertinent question (a lot of so called microagressions really are just plain old bad manners)