Whenever you join a mission with a large amount of slots, you'll find that you're unable to scroll down to reach the slots near the bottom.
As such making those slots unavailable for selection.
When you attempt to scroll down the game will instantly scroll back up to the top of the list.
Join/host a server with a large amount of slots available.
Try and scroll down to select a slot near the bottom, you'll see that the game instantly scrolls up to the top of the list making it impossible to select slots further down the list.
Also experiencing this issue. Only happened after upgrading to the RC from the beta.
I'm also seeing it happen in missions that utilize menus that require scrolling.