No.
Changing dns.dnssec requires FTL restart, but this shouldn't cause web session termination.
Yes.
Sessions should be stored on the database and reloaded on restart.
We are currently investigating another issue that a logout happens when the user changes CNAME records.
One of the recent FTL optimizations changed the order of execution of some things on FTL restarts. This is probably delaying the load of the previous web server sessions. When the delayed is longer than expected, the page will reload before FTL loads previous sessions data. Without this information, the web server will consider the session invalid and will redirect to the Login page.
There is already a fix in development branch to change the start order and load the web sessions earlier.