The contamination/dust that can be filtered when the rotary encoder speed is high may not be filtered at low speed, thus creating unsollicited direction change, etc.
At low speed, even a minor contamination may appear as a long pulse, and thus not filtered and processed as a standard quadrature encoder pulse.
This contamination can be detected by using the similar method as the missing index detection.
A contamination exists on a phase line if TC_SR.CPCS = 1 and TC_QISR.DIRCHG = 1 when there is no sollicited change of direction.