This is not a bug. It is by design. The eVic is limited to 2.5 amps, 5V or 11 watts. It reads the resistance of the atomizer/cartomizer you have attached and calculates what the maximum possible power is based on a 2.5A/5V/11W limit. This limit will change with each carto/atty you attach, based on its resistance.
The problem you are experiencing is with varying resistance. I have noticed this with the same 2.8 ohm rated head on my Vivi Nova. Sometimes it reads 2.7 ohms, and has read all the way up to 3.0 ohms. The maximum varies depending on what it reads. This is normal. If you have ever used a multimeter to read atty/carto resistance, it varies depending on where you measure, how far apart the leads are, etc. The same thing goes when screwing it into the eVic. It reads a little differently and adjusts power according to its current reading.
This is probably one of the biggest issues with its power limit. Of course, I personally don't see the need to go as high as 11 watts. Everything over 8 watts tastes burnt to me, no matter what carto/atty I've used on different variable voltage
devices (twist, lavatube, and now the eVic).
If you really need 11W, I suggest you stick in the 2.0 ohm range.