Just completed my first campaign as the Union, against Athena on hard. V1.16a, started the game before the 1.17 patch came out, but I don' think it affects this part: With much more offensive action on my part than when I played from the Confederate side, and with Athena being unusually aggressive herself, I saw an unreasonable number of implausible "retreats" where the retreating force wound up in the region from which the attack was launched.
First caught my attention in a big way in December of 1861, when Magruder launched a powerful assault on Butler's command (a strong division) on the grounds outside Fort Monroe. I'd neglected to give Butler a they-shall-not-pass ROE, so he tried to retreat almost immedatey. Result, Magruder takes Fort Monroe, and an only slightly weakened Butler winds up camped in James City.
I'm not complaining about losing Fort Monroe. My bad, trusting Butler's discretion. But it defies credibility that an army with its back to the sea, can escape destruction by "retreating" inland up the very peninsula from which the attack was launched. Worse still, there didn't seem to be anything left between Butler and Richmond, though I thought it would have been too gamey to exploit that.
OK, once is happenstance, twice coincidence, etc. So there was a marauding corps under Gustav Smith, cornered on the Delaware peninsula, who "retreated" all the way to Wilmington under repeated assaults. A force under A.S. Johnston at West Baton Rouge, pinned by an advancing Union Army (Butler's, in fact) and with Union warships closing the river. Defeated in battle, and "retreats" to Cajun, perilously close to a weakly-garrisoned New Orleans. Silly me, thinking an army solidly between the city and the only enemy in the vicinity would be an effective defense. And, fortunately only two turns before the fall of Richmond, Polk solidly encircled at Nashville, with active Union corps advancing from each of the four adjacent land regions and a solid wall of gunboats on the river. Grant wins the battle, leaving Polk only moderately weakened and parked in Humphreys, TN. With a straight shot at my logistical hub of Paducah. Other incidents along the same lines, more than I can easily recall.
In almost every case, a defeated enemy "retreating" into a region with 0% military control, from which an active corps-sized force had advanced on its position, and then positioned to tear into the attacker's rear by virtue of having teleported a mostly-intact division or two right over the front lines.
I read elsewhere that it was recently decided to retire the rule against retreating into regions with <5% military control; I suspect this is an unintended consequence of that change. I think I'd rather have the original rule, and take extra care ensuring my own safe line of retreat, than have to deal with indestructible enemy armies equipped with teleporters. Pretty sure I have figured out some strategies to win the latter game, but they are so gamey that I don't see the point - if I wanted to play a game with wholly arbitrary rules unconnected to reality, I'd stick with chess. As a simulation of an actual military conflict, this otherwise-excellent game fails severely on the basis of such impossible "retreat" behavior.
If you all are done patching AACW, then please put this one at the head of the list for fixing in AACW2.