I have tried a few different USB controllers and have the same problem as others with dropouts and stuck notes, using the camera adapter cable and a USB cable plugged directly into my Maudio 61es and key station 88.
The issue has to do with the signals coming from the keyboards and the data that the mini app must contend with. To pare down the signal to a simple streamlined transmission without garbage and other data the the app tends to choke on, I switched over to a midisport UNO 1x1 and an old Roland xv88 synth. Works like a champ, mostly due to the midisport, which seems to smooth things out, even for other apps that were acting screwy with the raw midi signals coming from the Roland when using other cheap USB midi cables. Midisport UNO was a good fix for me.
Would be great if someone finds a USB cable that likewise smooths things out, or if the app gets updated with a filter or more robust midi processor.
This works perfectly with this app, with my Roland keyboard. http://www.musiciansfriend.com/keyboards-midi/m-audio-usb-uno-1x1-midi-interface