I do not want to bash your sails but...
I know many racers who use NP racesails and complain all the time about front foot floating. Most of them just got used to that.
It has been a long time since my last NP sail (RS4) and I remember having the same problem with sonic100. It was a little bit less if I cranked the downhaul a LOT, but then the sails did not work very well. After I changed the sail, everything was perfect again.
I tried is86 with Vapor 6.7 and did not notice this problem at all.