I believe what's happening(and I've noticed it too with my 30a EVSE and my 6.6kWh Leaf charger) is the battery is near(or at 100%) so it doesn't call for a full 27.5a that the car can draw, it just takes a top off amount which unfortunately isn't enough to keep up with the preheat draw. The only way I've found to avoid this is to set my '13 to 80%(you won't have this option on your '14
) and set to 80% when the preheating starts the battery is low enough to basically draw the full amount the preheat is using from your EVSE. I know this won't do you any good if you want a 100% charge but it's just the way it is. Note you won't really lose more than a max of 3% off 100% from what I've observed but I understand when you want a full 100% you might feel cheated only getting 97%.
2012 SL purchased used 2/'16
2013 S w/QC purchased new
Juicebox Premium 60a L1/L2 EVSE, Ebusbar 16a L1/L2 EVSE
'12 EVSEupgrade'd 20a L1/L2 EVSE, '13 EVSEupgrade'd adjustable 6-20a L2, 6-13a L1 EVSE
Zencar 13, 20, 30a L1/L2 portable EVSE
GE Durastation 30a