I think you might be hitting the limit of the voltage based FOC control. For these kinds of velocities I think that misalignment due to not measuring the phase currents is becoming much higher and causing the rise of the current. And it is not clear to me why is it that dead_zone is helping to solve this (maybe it is just reducing the voltage you set) but I think without current measuring you might not be able to run the motor on such a high velocities with simplefoc.
We will be implementing the low-side sensing soon (that will be the next one we support). 