I've seen this in gameplays where the framerate is low/choppy due to the computer having bad or low specs. It's likely a hardware issue on your end, as I've seen gameplay from different players with smoother framerates not have this issue. Whats happening under the hood is there are calculations running to move the alien ( it's not an animation or something like that ) as it jumps out of the guy's mouth onto the floor. In slower computers, and with lag, the calculation goes too long/short or just not the right expected amount of time, causing the alien to overshoot the landing zone it was designed to hit. This causes it to miss the hidden trigger zone and the arrow as a result does not appear. Sorry that happened to you though!