Well, the only astrogeologist I know chose northern Europe, and the two nuclear physicists I know are doing their thesis in Zurich despite one of them having been proposed a subject by the MIT and a Singaporean university (I think he took the better subject tbh, probably didn't have anything to do with the current immigration policies in the US).

The admin stopping some research for no reason probably got a lot of PhD students quite distrustful of the US, that could cause some of them to go back after their thesis. Top PhD often choose countries for the lab director and the thesis subject. The US lost a few lab directors in the last 6 months, and at least in therapeutic engineering this year doesn't propose a lot of new thesis subjects (less than last year it seems), but maybe they keep them for their own students.