Most probably the Measurement Equipment manual was not followed. To avoid such accidents, a DC Switch between the measuring device and the string to be measured is highly recommended. PV Engineering have a nice on for their PVPM 1500X, but can be bought also such as the Zbeny. Then You use the switch to connect the string to the device and isolate it from the device. And/or have a clamp meter always on one of the cable and see if there is current flowing before opening any DC Connector or removing measurement pins.

To me, as long as it is in the stated tolerance declared by the manufacturer, it is no issue. You get a data sheet of what you want to buy and you as EPC have to make your calculations of yield based on the minimum of that tolerance. When it goes below that minimum declared by manufacturer, it is of course a different story. 3W below the declared nominated power is 0,6%!!! That is something normal, considering the 5% and 10% accepted by IEC for voltage and current. Besides, to detect a deviation of under 1%, you go into the deep world of Measurement tolerance due to measurement device tolerance...

@Călin Sas It is also hard to follow its progression, as it is highly influenced by weather. This also makes diagnosing the problems difficult. You have a Riso alarm on IT system and when you are on site, it's gone