The camera is 2 months old. It's very difficult for a K3 to be out of warranty
The button is being noticed by the software. I can see that much in the viewfinder - the 6-sided symbol is blinking.
It have tried it with C4/25 in both settings and get the same thing. I have now reverted to spot focus and still see the same issue but, subjectively, perhaps less often. Certainly I can easily reproduce a situation where I am holding the button down for many seconds, the symbol is blinking, it is in focus, but the shutter won't fire.