Actually, I don't think it is all that unusual for US citizens not to know where their ancestors come from in Africa, any more than Europe or Asia or South America once they are a couple of generations removed.

The demographics of the black population in the US has also changed considerably in the last 20 years, and the census reflects it. Many Caribbean immigrants, recent immigrants from Africa, etc. and they tend to identify their nations of origin as ancestry, as I indicated above.

It would be interesting to find out what % of the black population self-identifies as being African American descendants of slaves, and what % as not African American descendants of slaves. But they aren't given that option on the census the way Hispanics are given the option to be Hispanic/not Hispanic.