I order an AI team of 5 units to advance 100 meters, 4 of them advance, one stays behind unable to move.
Or they are in formation and I move ahead and 1 or more units will not follow and stay stuck at the same position.
This is so far random and I can not reliably reproduce this, but it happens quite often. It seems so far that in danger and stealth state it happens more (maybe because of the cover stances and positions they use then).