This was tested on 2.7, when I get the time later this week, I'll confirm on 2.8
In the meantime, can you post a screen grab?
Im only guessing, but have you given the POE a static IP??
Would the best use of mdk3 be on a VM using a wifi card to deauth a network, and then get the victims to connect to Pineapple invisibly.
My question is, what's stopping the victim's pc reconnecting to the original network once mdk3 finishes.