You are perhaps right, simply attacking from a city ungarrisons them...
I loaded a saved game, and first moved 3 units out, so I had the same number of units garrisoned as I could fit in my army. I wasn't picking a subset; it didn't matter, they would first leave the city by standing on top of it. Thus, even though they end their turn on a city tile, they aren't in the city anymore.... I guess that's a feature, they've left the city walls..., so they end turn outside the city.
Well either it is a bug (and they should be re-added to the city defenders upon winning the battle with the adjacent enemies) or it is a feature, and some effort to make the 'outside the wall' guys on top of cities easier to see might be nice... Only the Stardock designer guys can tell us for sure...
I can really see this both ways, and understand the frustration. Especially where we don't have true siege rules, where it's kind of hard to replicate firing from behind the safety of city walls and such... BTW, I'm more inclined to think that this a bug, IMHO.
Stardock guys: If this is indeed NOT A BUG, please be so kind as to make sure this is mentioned in the documentation under movement in the final release. I can see this frustrating more than a few players.
And if it IS a bug, well we wish you well in your efforts to squash it!!!
P.S. I'm enjoying what you've all done with LH. Almost as much fun as the first time I played the orginal E:WoM...
Nice job!