You definitely want to be using 6A+B and 6A+B BE against Nightmare. It takes away most of his stance game and also TC a lot of his horizontal high attacks. The more Nightmare rushes Maxi the potentially easier the matchup becomes.
I've also found that 6A+B to be effective against Natsu. A...
I saw something similar, but with 6B. It's happened to me 2 or 3 times since the patch but I don't recall it before (and I haven't been able to do it in training).