Fleeing the mat can be a hard call. A simple if they are facing out of the circle when they leave isn't good as a per se rule. If they are already on the edge, the best move might be to turn out. It doesn't mean they are avoiding wrestling, but the best defense in that case results in what appears to be a fleeing the mat situation. It is definitely undercalled I think, but I think that happens with all of the calls that are heavily subjective and making the rule more objective isn't good.
More than from neutral I would like to see it called when the top man runs the bottom guy out of bounds after a standup.