Hi Benjamin,
All depends on context, of course. I would say it is quite tiring to answer a question that has no easy answer to everyone we meet. I am sure if it were asked only once in a while, it would be no burden.
Sometimes, I too, find myself being genuinely curious about people’s heritage, but I prefer to wait and let them share their story on their own terms and in their own time
If you would like to ask, I think asking “What’s your story?” works the best. Here, you are interested in the person, rather then whatever mixes of races they are.