Linearity of stall torque versus motor current

I am currently working on a project to estimate the stall output torque as a function of the input motor current using Odrive. The motor that I was testing right now is Turnigy Aerodrive SK3 - 6364-245KV. In addition, a gearbox with ratio 16:1 has been installed in the system in order to increase the overall output torque.

Without the introduction of gearbox, a linearly correlation between motor torque and current can be well observed when the current is gradually increased (0A to 40A) as well as decreased (40A to 0A). However, this is not the case when the gearbox is introduced. The output torque still rises gradually with the increasing motor current (0A to 25A). However, when the current is decreased stepwise from 25A to 0A, the output torque remain constant initially and dropped abruptly when the current is reduced to a value below 5A.

Has anybody experienced such hysteresis issue? Has it something to do with the backlash of the gearbox? How can this be resolved? Bigger motor with gearbox of smaller gear ratio?

Little can be said without knowing the gearbox and torque measurement setup. That being said, my first suspect would be mechanical jamming that would leave the gearbox jammed in the high torque position till most of the input torque is removed.