Yes defo good that they have released a new one.
Unfortunately (in a wider/general sense, not about this car seat specifically) failing a consumer test like Which/ADAC doesn't always mean a car seat will be taken off the market, no. To be allowed to be sold, seats must pass the legal regulation, of which there are two running at the moment, but both of them involve a crash test at about 30mph. All the seats tested by Which will pass this legal testing, if they failed it they wouldn't be allowed to be sold. The tests that Which do are at a higher speed, about 40mph, and the main useful thing you get from these tests is info about specifically how the seat performed. The legal standard doesn't give you that, it's just a pass/fail. But very occasionally you'll find that a seat which passes the legal standard fine catastrophically fails the Which testing. Very often the seat is then recalled and modified and if the new version is later tested it does fine, but sometimes the company will release a statement that (basically) says "But it passes the legal standard, so we don't need to recall it". The seat will often still be withdrawn because of the bad publicity but sometimes it's later quietly re-released with another name. This most often happens with generic type seats, for example there was one a few years ago which was part of Argos' own brand budget line of car seats. The seat was withdrawn from sale, but if you go to Argos' website right now you can see an identical car seat on sale with a new name. In fact they have it in about four different incarnations. They have renamed their entire budget baby line. And if you go to the Asda website or Halfords, you can see it there with another name again. It's also one of the most widely used seats in the UK, which is bloody scary. I think as they are cheap and sold everywhere, people buy them as a spare or generally to use, thinking the more expensive models are a waste of money - they are not.
The babystyle carapace seat did really well under seatbelt mode, so I don't think it was even a bad seat in the first place, maybe just a faulty connector or something - but in general it is a good idea to do your research about car seats and defo never buy a cheap generic one or one from a supermarket. I'd note this applies mainly to the toddler stage seats, infant seats generally do well even from manufacturers with poor track records. The only time they tend to fail is this kind of error - detachment from a base, and that is rare.
Sorry 
you can go back to pram talk now!