His pull being able to wrangle you back in is from the same mechanic that allows your friendly neighborhood Ezreal to escape Blitz grabs and Lux to continue ulting despite CC: Cast times.
At the beginning of the pull, you're targetted by the pull to be pulled to a location directly in front of Darius. If you flash mid-pull animation then you will be brought back because while Flash is instant and that's cool and all, that means it does not override the targetting mechanism of Darius' cast timed pull because Flash already got you where you needed to go. After that, Darius is still casting, and so at the very end of the cast, you go to the location marked for you since you didn't have an overlapping cast time that would continue after Darius' to get you out of there, like, say, Ezreal can. With Ezreal's blink, because it has a cast time, everything about Darius' pull applies to his blink, except instead of bringing enemies already marked by the ability to Darius' face, Ezreal's blink will ensure he ends up at his targetted destination.
In short: It's not a bug, it's a feature at this point. Unless the cast time is removed, it will not change.