They are offering to replace it with the Minikid 4 Pro, as far as I can see (unless that is only in Germany?) The Minikid 4 Pro has tethers, so it will be more secure than the Core, and in the previous version of the ADAC test this seat (under an older model name, Movekid) did extremely well. In addition, the Core performs well in all modes except when used fully reclined with the heaviest dummy it is approved for - which is a really unrealistic use of the seat. Usually with the Minikid seats, you only use the reclined position with younger children.
To be honest I find the results of the ADAC testing (which is the testing that Which buy and use for their reviews) very strange since they changed their test criteria in 2025. It reminds me of a test that was started in 2021 in Norway (by NAF and Testfakta if you want to look this up, though you'll need google translate) which was meant to be a sort of version of the Plus Test but for infant carriers. It had really strange results and ultimately it was criticised for being too severe and not realistic, was never repeated and is almost impossible to find info about it on the internet without using the Wayback Machine. It's possible to increase the severity of a crash test by increasing the "pulse" which basically means shortening the distance that the vehicle has to stop in - essentially, the difference between crashing into something like a crash barrier, which is designed to deform and slow down the car more gradually, or a solid concrete wall, which does not. This is part of why the Plus test is so strict, and part of what NaF did to test their infant carriers, though I can't find the exact info any more.
One of the big changes with the 2025 ADAC test criteria was also exactly this - they halved the stopping distance, and they crash the test car into a solid wall rather than a deformable one. So the test is very different to previous years and it is not possible to directly compare the results. It reminds me of the NAF test because of how many seats from extremely well respected brands, brands who have built an entire long reputation and business on the extent of their own internal testing, performed badly. Did you know Britax test all their isofix seats with only one isofix arm attached, and if it fails, they redesign the seat until it does not? It just makes absolutely no sense that a company who is designing for this level of redundancy is failing a general consumer crash test, unless the criteria for the general consumer crash test is not well designed.
The other reason it reminds me of the NAF test is that in the NAF test, IIRC, all the seats that failed did so because of "high chest loading" (which means large amounts of force impacted on the crash test dummy's chest area) and this seems to be a pattern as well with both the Axkid seats which haven't done well (and both only in the reclined position) as well as the Britax seat (which was also in the reclined position). And again you'll see things break within the physical seat itself when the test pulse is higher. It kind of makes me wonder whether ADAC are optimising for those videos that get such traction on social media because a part of a car seat or crash test dummy is flying through the air.
The thing with crash testing is that you can destroy anything if you treat it violently enough. People on car seat groups like to exclaim about the "low speed" of legal limit crash testing, but actually testing seats at 30/40mph gives much more of a sense of how the seat is likely to perform in a real life crash, because the vast majority of crashes do occur at these kinds of speeds, and even crashes on higher speed limit roads often have a lower actual speed at impact, because people usually brake when they think they are about to crash.
There isn't a huge amount of point in simulating extremely severe car crashes in a crash test lab, because once you get to extremely severe car accidents, the movement of the vehicles is much less controlled than it is in a crash test lab and there is often intrusion into the passenger compartment as well which makes the performance of features like child seats less relevant to overall survival. But also, because a crash at 30/40mph is still an extremely violent impact, if a child seat is designed to protect the occupant during a 30-40mph crash, it's still likely that it will have some protective effect in more violent crashes, it's just harder to predict. Whereas products like the fabric car harnesses, or baby slings, or dog car harnesses which are not designed for crash safety at all literally fall apart in even the most minor crash test situation.
I have seen comments on social media claiming that the Axkid Core was wrongly installed which is why it failed - I don't see any evidence backing this up, though, and I can't find any more reliable source so I would remain sceptical about these claims, unless anyone knows what the source is.
My suspicion is that the seats genuinely did fail the test criteria, but I do not know if I trust the test criteria to tell us much that is useful. I hope there will be more commentary from people more informed than me because I'd be happy to learn more about it. I can't find any actual figures about what ADAC changed, everything talking about it is frustratingly vague.