I've not come up with a good explanation of "kit taste", but it's a harsh flavor that override's the wine's normal flavor. IME, all wines with kit taste have the same off-flavor.
When eliminating all other possible causes, kit wine taste appears to be connected to the concentrate production.
@Matteo_Lahm states the problem is solved by not pasteurizing the concentrate -- while I lack the knowledge to say that is correct, the fact is that FWK's process works to prevent kit taste. I'm not worried about the "why", just the "what", so I accept his statement as correct.