If a unit is in a group outside of a city, its movement is limited that turn to that of the slowest unit. This is true even if you eject the units from a city and then move them back into the city. It has been like this since I've played.

Furthermore, if you move a unit into a group (or out of a group and back into the same group), the group's movement will be updated. So, if the unit moving into the group uses its last movement point to join the group, the group won't be able to move farther.

While some have complained of this behavior, I'm pretty certain it is working as intended.

I believe the independent tallies are kept because if any movement points are left you can always split the group.

And sometimes it's worthwhile to split a group and send everyone on their own scouting mission.

And sometimes it's worthwhile to split several groups and re-merge their units -- arranged differently.

And note also that sometimes you can use this mechanism offensively.  If you web a unit in an opponent army, that army is stalled for that season, regardless of the outcome of the battle.  (Obviously, this is a cheaper tactic if you use the "escape" air spell than if you have to sacrifice a unit to achieve this effect.)

(The painful part is that if you are maneuvering your army one tile at a time you cannot stack your units if they think they can merge.  So sometimes you need to think very carefully about how you perform your maneuvers.)

Are you saying units during your turn, units that you are not actively moving, and which are not receiving other units have their available movement decreased?  That does indeed sound like a bug.  To my knowledge, decreases in available movement during a turn are supposed to be caused by doing something: movement, combat, receiving other troops.  I have never seen movement decrease when other units leave the army.

Ok.

That is surprising -- I have not seen this issue.  When I move units out, I have never seen the remaining units change.

I imagine stardock's people will want to fix this (once they are back from vacation, and if they understand how to make it happen -- I do not know how to make it happen myself, so I cannot explain that to them).

I would say that woudln't be working as intended; or is a bug even if what they intended;  since it goes against what players expect to happen, theres' no need for it from a balance perspective; and games as old as the original warlords and probably even older had it working right;, so it can't be that hard to do.