If you read the description GRP tends to go to the closest subject among the 4 sensors, but it will also be the one with most contrast. You have no control over what it picks and it may pick the wrong thing.
The DOF is what it is regardless and all you do is pick what to focus on. DOF at macro focal lengths is said to evenly distribute in front of and behind the focal point, so focusing on the front of your subject wastes DOF. So best approach is to focus on the middle of the subject to give best chance of getting the DOF to fall in the right spot. To do that you really need to use a single focus point.
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