That's the problem with the way the US does race and ethnicity statistics. Self-reporting.
Apparently the moment they start to be born in the US, they turn white xD
No need to suspect anything. We have loads of data on this.
https://www.pewresearch.org/race-and-ethnicity/2021/11/04/measuring-the-racial-identity-of-latinos/
Discussion
They might be white and something else. Before the 70's there was no differentiation for Hispanics, so it might be more accurate to say that they all turned Hispanic in the 70's.