I finally got around to having another look at this. Running the latest version of Shadows, the postures almost work.
Defensive postures sometimes do work correctly, other times they don't. When they don't work (i.e. there is a visible enemy ship in range of the posture but the defensive fleet does not move), the stance of the ships in the fleet will reset after a moment if I try to change them - as if the ship is being assigned a mission with a different default stance. It looks like the ship is being assigned a mission over and over again, but the ship can't/won't respond to the mission.
The Attack posture is also doing better than before - I put two enemy mining stations in a system, and set one of them as the posture attack target with sector range. As before, the fleet successfully destroyed the initial target, however this time it left the posture intact (previously it always deactivated the posture). Right now the ships are back in their home system exhibiting odd behaviour. Every few seconds the ships/fleet are assigned the mission to attack the other mining station which is in range of the attack posture - however the mission is immediately cancelled. At the same moment as the attack mission is assigned health/shield bars appear over the ships for some reason.
The really interesting part is that as with the defensive posture ships any stance I set is reset to one particular one - but not by the assignment of the attack misson. The attack mission sets it to 'nearby targets', but something else is setting it to 'system'. So if I set a ship's stance to 'only when attacked', I can watch *something* change the stance to 'system', then the posture mission kicks in and changes it to 'nearby', and then the *something* kicks in again and sets it back to 'system' again.
It looks to me like there is some kind of 'ghost mission' getting assigned over and over again and blocking the ships from carrying out their posture duties, some of the time.