Hmmm. I wonder if it's the ap2000 that's the problem. It toggles the busy and ack with very little delay between them. According to the LX-810/850 service manual the ack/busy should last for approx 12 us. (ack for 12us total, with busy dropping at the 7us mark).

Perhaps there was circuitry inside the gate array that created the delay for this timing.