We have observed this problem, too.
Sporadic StallGuard excitation occurs under certain circumstances without the motor beeing blocked. This is a firmware bug, which we will eliminate within the next few days. Please watch out for updates.
This problem is solved in Version V3.30 (respectively V6.22), now available for donwload!
Cirumstances under which the wrong StallGuard values occur on single axis units (TMCM109, TMCM110, TMCM111):
When the microstep frequency during a StallGuard homing is higher than about 20kHz. To avoid this happening, you can reduce the microstep frequency as a work-around.
Example: For the homing with StallGuard use a setting of 4, i.e. 16 microsteps, instead of 6 (64 microsteps), which is the default for most PANdrives.
Cirumstances under which the wrong StallGuard values occur on multi axis units (TMCM3xx, TMCM6xx):
When the microstep frequency during a StallGuard homing is higher than about 20kHz, or when more than one motor within 3 motors runs at a time.
The work around is as for the single axis units. Additionally, stop the other motors controlled by the same TMC428.