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.