After meeting Daf, I went to Wales and visited the factory. A string was made before my eyes and having seen that, it seems obvious that the kink is inevitable and that Daf's explanation is right. Because the core is not metal, it will be much more flexible than steel core strings.
When the strings are wound, the core has a quite thick sticky stuff around it. I suppose that this glewy substance gets cracked under tension, but since it has no part in the string resistance - given exclusively by the thread of the core - there is nothing wrong.
And since the new version of the braided strings will have a core which is even more flexible, it will appear to be worse when in fact they will be all right!
So why did the sound go wrong on my string? No idea, and now that Daf has shredded it, we probably won't know.
People at the factory were extremely kind and it was very interesting. Great place!