MIDI is a serial protocol, meaning nothing is sent through MIDI at the same time as anything else. Regardless of the sequencer you are using, sending 64 note-on events takes enough time that the delay is long enough to be perceived. If there is pitch bend, aftertouch, or too much other stuff being sent, that makes the problem even worse. Even one delayed note is enough to throw off your perception of the rhythm for several measures. The problem could easily be someone's sequence rather than their equipment; I wouldn't take seriously claims of hardware problems unless someone showed that the same sequence plays back fine with another sequencer or another synth of the same polyphony. One great thing about the XP-80's sequencer is that it makes it very easy to thin or erase aftertouch and other controller messages to avoid just this problem. You will have to look very carefully at your 64 part sequences to get them to play back right on any MIDI equipment.

I can't reproduce the problem on my XP-80, which is only a few months old. It looks like Roland has updated the ROM to take care of some specific timing issues they noticed.