I hate to draw attention back to my theory but if in RAWs Sanji still didn't flip his eyebrow. It means, yes he is purposefully not doing it since his fight with S-Shark
Something happened.
I still say he scared Nami in that fight and he has been actively avoiding REAL combat since. (Bubble Kaku, used one hand instead of getting close to Nami to grab her gun to do it something abnormal. Let Zoro fight for Stussy, has only dodged, defended, or tried to save people and avoid getting into combat.)
Look at the signs. His actions to his personality have been abnormal since then and he hasn't switched into battle mode he so easily decided to use against S-Shark.
He scared her and is now holding
back.