another thought, I have never used these drivers however the config. h file contains the note
// Stepper driver models (also see ~/OnStep/src/sd_drivers/Models.h for additional infrequently used models and more info.):
// A4988, DRV8825, LV8729, S109, SSS TMC2209*, TMC2130* **, and TMC5160* ***
// * = add _QUIET (stealthChop tracking) for example "TMC2130_QUIET"
// ** = SSS TMC2130 if you choose to set stepper driver current (in mA) set Vref pot. 2.5V instead of by motor current as usual.
i have no idea but it may mean that if you enter a current in the settings it get scaled by Vref i.e if you don't have it set to 2.5V but set 0.5V you might get 1/5 the expected current.