Having some more data!
First, although a gut feeling hopes it does, I'm unsure how much or if at all relates to events described by Nomad. So please don't hesitate to cut me down to pieces and disperse.
Some common factors:
-- Missions with Mafia in my case (Ray's, Blackmail), but that's possibly just coincidence.
-- Units in HtH mode during movement. It started actually happening more often once switched whole party into HtH. So perhaps factor, but possibly we simply moved more often...
So various things occur:
1) An ally taking less steps than requested in a turn. It can auto-end turn. It's tricky to compare, in the log I may have seen something as if the character was attacking somebody or at least requesting CTH estimation, before reaching destination. Probably nonsense though.
2) An ally taking completely different path than requested. For example, take a look at this picture:
S - Start
X - Requested destination
At the beginning, Sam nicely steps south as requested but then completely changes his mind and eventually ends at the position where we see him at the picture.
3) If there is enough AP (3) an ally can decide attack a nearby enemy (often in combination with taking different path). So just a turn before the Sam's running at loose I've got Elena disobeying simple polite move request in favor of punching the mafioso:
Note, Elena ended slightly displaced on that tile, but there was no problem with this afterwards.
As I play Vigilantes partially as text adventure anyway, as soon as those two above happened, copied the last content from the log regarding their turns. So it's attached in case it helps anything.
Could be wrong, but it seems as if the path finding is initialized correctly in the requested direction at first, and often the first step goes well, but then the game takes the reins and changes orders. As if the AI is at play or so...
I knew I shouldn't have dumped the Leadership!