This applies from 7th October 2021.
On the first Thursday of each month, security and feature updates are applied to the CentOS Linux VDI systems.
This takes place sometime after 19:30 UK local time (and just before the Windows VDI updates).
Examples of the updates applied are non-critical security updates, and necessary reconfiguration to allow features to still work.
In general, software is not upgraded unless necessary and unless patches have been released by official upstream maintainers (CentOS, or ECMWF, for example).
After the session you will need to log in again using graphical access (Horizon client or browser) and note the hostname, which may have changed, for future SSH connections.
If you try to log in using your old system hostname without the graphical step, it might work, because you will connect to someone else's VDI session!