When this happens, do you see any death messages at all? My guess is 1 of 2 things is happening:
1) Your "msg" setting is set too high and it is skipping all death messages. The client side scoreboard needs these messages to adjust the scores on the fly.
2) During your connection to the server, your userinfo data is somehow getting corrupted or truncated, so it's not passing the build number correctly and the server is treating your client like an old Quake2 client, which doesn't have a client-side scoreboard.