EDIT: Was able to test it just now. I found out that my specific issue was that i didnt know the c stick could be used as a directional input AFTER using it for a special move (with c stick set to special) So theres no bug or other issue. So end of the day i cant say i have your issue and can only guess that maybe you hit the c stick after doing your up b, or you hold a direction with your analog stick right after doing up b when you dont need to.
Ignore everything written below for discussion purposes.
Sometimes the aerial up b, and even the down b, will b reverse or wavebounce for no reason. You can try this with c stick set to special and it (the bug) will be especially glaring. It only happens once or twice and then you can't recreate it right away. Sometimes my thumb ISNT EVEN ON THE ANALOG STICK and I only use the c stick to do up or down special, and it b reverses. Keep in mind that b reversing involves you doing the special, and then flicking the analog stick. So when i'm only using the c stick but not the analog stick....like...wtf?
My assumption is that it may have something to do with buffers in this game IF its not a bug of some sort? I honestly dont know. I'll play around with it tonight trying to show where i find it buggy and report back.
and this may or may not be related, but I have a very difficult time doing a b reverse ASK when DK is moving while facing the opposite direction in this game. Like, it's close to impossible to do it consistently. I've tried all variations of it where he b reverses or wavebounces and its so ****ing difficult.