I have no experience with Maya and I fought someone who beat me using nothing but that air lunge attack.
Its clearly unblockable, so I tried to counter with Roundhouse but I dont know when theyre using it or not. Do they have to jump vertically or can they do it while jumping forward?
also my character doesnt have the best dp.
They can do it from a neutral or forward jump, but not a backwards jump, and they must have both daggers. Crouch to make it automatically miss. 
In case it isn’t clear enough, mantis can’t connect with a crouching opponent. Instead, Maya will tumble in front of you, leaving you with a generous grounded punish opportunity.
It’s worth checking the in-game move list, as well as Infil’s guide, whenever you come across a move that is getting the best of you, to see if it has any obvious counters.
really? I never knew this! thanks guys! Glad I read this thread… now I know to crouch on those Maya spammers