How are we in version 5.2.4 and besiegers keeps retreating INTO the fort ?

I remember it was a pretty important critic in the early days of Warhammer III. I left the game for some months, expecting it to improve over time, to see most of the bugs and problem fixed by the time I got back. Tried. First siege as defender, and I saw it. Besiegers keep retreating inside the fort. Of course, I have other things to deal with during the siege, so the ennemies retreat, I stop bothering. Only to see them charging my archers in the back because they reformed 10 seconds later. I just can't understand how this problem isn't fixed. It was a problem day 1, and still is a problem years later ?!