It's possible it could be an incomplete fill, but I would bet on an assembly line, it would be charged quickly and precisely.
In order to properly diagnose the problem, you need to know how much was in the system. Then either fix it, or add some as you have done, which could also fix it.
You are looking for.
Stabilized system pressure relative to ambient temperature and humidity.
System pressures within specs under load (operating)
Vent exit air temperature under specific conditions, again relative to ambient temperatures and humidity.
If it does not pass these tests then extract and weight the fluid to see if it is the proper amount.
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View my fuel log 2015 Mirage de 1.2 manual: 55.7 mpg (US) ... 23.7 km/L ... 4.2 L/100 km ... 66.9 mpg (Imp)