@WISdad23 If you check the data more broadly, you will find that exactly the same is true for many private universities, such as Harvard, Yale, Princeton, MIT, Stanford, and Johns Hopkins. All of these schools have “downward economic drift”, as you define it. This is most likely because they start with such low numbers of students in the Bottom 20%.
Conversely, the most impressive examples of “upward drift” appear to be at regional state universities that start with high numbers of students from the Bottom 20%. For example, Cal State Dominguez Hills gets 22% of students from the Bottom 20%, but only 11% of graduates are there at age 34.
I unreservedly salute the work done by schools like CSUDH or Texas-El Paso. But given the choice, I probably wouldn’t recommend them to a student over Princeton or Bowdoin, regardless of the “drift measurement”. Of course, that’s just one subjective opinion; others may disagree.