G-sensor is hardware-based, there is a part in each cam doing the activation. There can and will be variations in those parts, so there can and will be some variation in how well it works in each cam. Motion-detect is software-based, when there's a change in a certain amount of pixels in the sensor's view it activates. Thus each and every cam should behave near-identically with that as only sensor variations can affect it, and those are minuscule. I'm using the same 'software' as everyone else, so I cannot understand how anyone gets a different result than me- it shouldn't be happening.
Over time one of the things we've learned about M/D is that changes in light, blinking LED's in a cam's view from devices nearby or a car's security system can 'trip up' this function, activating it unexpectedly. Especially the blinking LED's can be hard to deal with at any sensitivity setting. Changing traffic signals can do the same thing. The cam doesn't have to directly 'see' these sources; they can be reflected off the glass and still cause problems. Anything changing is any part of any cam's view can activate M/D if it's significant enough.
So before dumping M/D as not working check carefully for activation sources such as these, remembering that with the A139 every cam is detecting, not just the front one. If it's an in-car activation source you'll see similar results no matter where you park; if it's an outside source activating you'll get varying results. If possible test it in a 'sterile' environment such as a closed garage or parked somewhere on a cloudless day where there will be no kind of movement around or in the car. Try it with the rear cam unplugged then with just the cabin cam unplugged. At some point there should be an indication of what or at least where the problem is, then maybe there can be a fix.
Phil