The goal for Mystic is to trade with ex push in order to further combo the opponent. Though again, to trigger this is to trade with specific moves on a very particular frame. I'm not saying it's impossible to do. I've done it over a dozen times in practice mode. However, the chance of purposefully landing this in a real match is rather small. Let's assume it has been in the game since launch. This would mean that in a 1 year 1 mo we've seen this occur once. Granted Mystic hasn't received much use from the community since the XL patch that point remains the same. If we are to assume that the glitch came with XL, then it's been roughly 2 months and we've seen it done once. If it were that easy, then we'd likely have seen it done before now. By Toxin's own admission, out of 40 games with Red Raptor and countless trades with ex Kunai this glitch occurred but 1 time.
A ban is not necessary in my opinion.