People should really refrain from posting in the bug forum just because they don't understand something. This isn't a bug. Pittsburgh signed it's new player, putting their roster at 54, on the day of a game sim. When the sim occurred the AI cut a player to bring the roster back down to 53. (In this specific case the player cut was the one who had just been signed.)
Don't sign players to your team on game sim days (regular season or post-season) unless you've made corresponding cuts to keep your roster at or below 53 men.