I believe I have this same exact problem, if I'm understanding the OPer's description. I've been tweaking settings trying to correct it, but with no such luck. My HUD is set to 100%. It is true, as someone here has mentioned, if you lower it then you completely lose the 5th champion (4th champion even if you lower it all the way to 0.) Even at 100% though, I can see maybe half of the 5th champion on the scoreboard. It's not game-breaking, but it is rather annoying.
http://imgur.com/a/Di2Da
This is the issue in spectator mode. I have another similar issue. When I'm actually playing (not in spectator mode), almost the opposite happens. The top part of the screen is slightly cut off. I only see about half of the score/timer in the top right corner. If I go into the settings and switch from "fullscreen" to "borderless", it corrects this. It does not however fix anything in spectator mode regarding the scoreboard.