the "flip flop" from positive pitch to negative pitch due to slop in the rotor head is what causes this

if you have a high enough head speed to maintain a hover with very little pitch, as soon as you go near zero pitch, the linkages allow one of the blades to 'flop" over to negative and get that sudden, powered decent

the only way to alleviate this in a bird with the AXE's slop, is to increase the pitch at lower throttle, so you will descend well before zero pitch due to lack of head speed

or get rid of the slop with a metal center hub/flybar system, ala microheli

the other, less likely possibility, is that you have your flybar paddles on reversed