DNS TTL is not honoured. It looks like the client cache's DNS entries for the entirety of the game session. Turning this off may cause issues for featured servers so it would be within interest to contact the featured servers to see if this will incur them a lot of extra cost.

To do a fresh lookup the client must be completely restarted.

This can be validated with wireshark and filtering by dns. You'll see that there's initial lookups when opening the server list, but after that lookups do not happen unless you're adding a new server.