I'm connecting my iRig keys into a 2017 MacBook Air and I'm getting the same problem in Logic Pro X and GarageBand ... when I play a bass through midi the sound squelches on a fast run, or just goes a bit out of tune, like drops down a semitone.
I just tried opening a brand new software instrument track on an empty GBproject, chose Fretless bass, played a note and it dropped a semitone, then changed the input to piano and got piano AND bass. Then changed to a drum kit and got drum kit and bass, but only at the lower end of the keyboard.
I've tried a factory reset as in the iRig Keys manual and it doesn't make any difference. I'm using the correct lead and going clean into the USB port.
I can only conclude that the iRig Keys has somehow gone faulty, though I've hardly used it since I got it 18 months ago.
Help please?