B737 MAX 8
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 240; Flight Crew Type 13000
Confusion; Distraction; Situational Awareness; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft
The Autopilot A was engaged. The RNAV ZZZZZ3 Arrival and the RNPZ XXL were programmed in the FMC. The  approach was not connected at ZZZZZ; as we were not cleared for the approach.  Approximately five miles prior to ZZZZZ1 [fix] we were told to expect vectors for the ILS. We selected the ILS XXL in the FMC; and upon executing the selection; we received an Autopilot Disconnect message; and an A/P Disconnect Caution light. The aircraft went into Control Wheel Pitch. I; the Pilot Flying; immediately selected Vertical Speed to keep the aircraft in a state towards the impending crossing restriction. I also reset the Altitude Alerter for X;000 feet; as we now were in Vertical Speed and needed altitude protection at ZZZZZ2 [fix].A few seconds later we passed over ZZZZZ2. The Autopilot Disconnect Caution light illuminated; and the Autopilot Lateral Mode switched to Control Wheel Steering; and the aircraft didn't turn and follow the still illuminated and displayed LNAV PATH. I immediately engaged heading mode; and turned to rejoin our intended course. Approach gave us a heading; as the minor deviation from our path was most likely noticeable.  The rest of the flight proceeded uneventfully.  We assumed we programmed the FMC incorrectly; in some way; and we needed to be more vigilant.  ZZZ to ZZZ1 [airport]; we were on the ZZZZZ2 RNAV; Autopilot B engaged; on PATH. We were given an intercept heading to join final for [Runway] XX. Heading mode was selected. The aircraft flew heading and was in VNAV. The final approach course was entered into the FMC; and upon executing; the Autopilot B Vertical mode disengaged; with the A/P Caution light and the aircraft descended in Control Wheel Pitch.  We selected a pitch mode and proceeded without further incident.My concern:  I don't know why these disconnects happened. If they are normal for the -800 MAX; I am unaware of this. I thought all of our fleet types were programmed and responded to Pilot programming the same; in this scenario.  I've never experienced [other model] aircraft revert to CWS Pitch or roll; even with faulty Pilot programming.  We both assumed that perhaps; since neither of us had flown a MAX in a few years that we missed some training; or company notice.  We assumed that the error was a Pilot programming error.   I called the Chief Pilot on Call and told him of the occurrence the first time; and he was unaware of such a thing happening normally. We confessed that we actually had more experience than him in the MAX. He indicated I should call later and talk to the MAX programming expert and he perhaps could explain why the MAX behaves in this way. That is why we flew the aircraft a second leg with the anomaly; I assumed it was Pilot programming error.  A few days later; I was contacted by a Chief Pilot; who informed me that our experience was a malfunction; and that a logbook entry and maintenance resolution should have taken place.  I assumed it was a MAX anomaly; but it was a malfunction.
Air Carrier pilot reported two instances of the B737 MAX Auto Pilot malfunctioning that the reporter did not enter into the maintenance logbook.