This is a solved problem info that could help out other users if for any reason they plug the 3DR Power Module in the wrong direction.
The current flow is indicated on one side of the small PCB with an “IN” arrow and an “OUT” arrow besides the positive soldered ends of the power cables. This is pretty good. However, if one looks only at the other side of the PCB, she/he may find another marking that can become misleading: we see “BATT -” with an arrow pointing at the negative soldering point to the power cable. This can mislead someone to think this wire goes to the battery side of the power module, which is simply the wrong way to connect it.
Normally, XT60 connectors are already soldered to the power cable ends at both sides and the male end is always where current comes from. Just check if this configuration has been followed. For an unknown reason (either my mistake soldering XT60 connectors or a factory mistake), I had a reversed direction Power Module plugged in. In this case the BEC works fine and provides +5V to the flight controller, you get correct voltage reading in Mission Planner but you cannot have any reading of the current (Amps) measurement and therefore no info on the remaining batteries capacity.
So if you have no Amps reading in Mission Planner and the rest goes fine while having all parameters configured correctly, you have either a bad wire connection on the 6-wire connector to the flight controller or a Power Module plugged in the wrong direction. Simply, the 6-wire connector on the Power Module goes on the same side as the flight controller.