Spending couple of days via email with OMC technician to trouble shoot and root cause finding. The technician concluded that the M6x20 cap screw is causing gearbox deformation which might be irreversible. Below is the photo what OMC technician email me:-
By using 4 M4 cap screw and washer between J2 Motor Support and J1 Platform.
AR2 has been running 6 hours (yesterday) and 4 hours (today, now still running) without J2 motor gearbox problem, I will update if problem keep come back which J2 motor gearbox deformation already irreversible and have to replace with new one.
M6X20 cap screw and locking might not needed in aluminium structure which original is for 3D printed.