Why assume it would have anything to do with the scientists themselves, and not the end of the russian funding which had been supporting them?

Most non-EU people, whether russian or american or zendian, have residency tied to employment, so end of contract equivales end of residency.

https://home.cern/news/news/cern/cern-council-decides-conclu...

> Relations continue with scientists of Russian or Belarusian nationality otherwise affiliated with CERN.