Is it okay to pivot on my lead leg while throwing my jab? (Kickboxing)?

What your instructor is saying is that by pivoting slightly on the ball of that foot you are exposing the back of your leg and knee more to damage from leg kicks and rather than doing that bend or flex your knee a little more which is how I teach my students. You can also turn you hip slightly behind your jab when doing this for power and turning that leg and foot to far makes you more susceptible to other things much worse than leg kicks at times as well so I would certainly consider changing it especially if you are going to fight in Muay Thai or K1 type fights where leg kicks are legal or if you seem to be getting hit by some things like overhand rights a lot.