Some explanation from the website of DarkGo’s creator, pjreddie (who also administers the bot here on OGS). It appears the bot does no search (contrary to AlphaGo, Leela and many other bots), so it judges what to play purely on the current position on the board. This makes it very vulnerable to ladders, of course.
It seems to have trained on professional games, so if the problematic joseki has never been played by profs, that might be why DarkGo does not recognise its pitfalls. Animiral gave a pretty good explanation in an earlier forum post over here