It would be very helpful to see a screenshot of your navigation route in the menu so that we can try to reproduce it.
i guess I´ve discovered what the problem is. This is a very sharp turn (nearly 180 deg) so the LNAV starts turning too early and that messes up the route because it never reaches the waypoint, won´t continue the route and finally disconnects. I´m not criticizing the aircraft, maybe this is not a turn to be done with LNAV engaged.