Volkswagen Passat Estate (2015 - 2024) 2.0 TDI SCR (190bhp) GT 5d Owner Review

Volkswagen Beware! VW "through corrosion" warranty not honoured.
My 2015 Passat has a number of corrosion blisters under the paintwork on the tailgate. One of these has broken the paint and rusted.
However VW will not honour any warranty claim.
The VW dealer warranty manager has confirmed the problem as Zinc Oxidation. VW acknowledge this. They claim Zinc Oxidation (a type of corrosion) underneath the paint finish is not included in their "12 year through corrosion warranty".
There is no technical explanation when purchasing a car that the through corrosion does not apply if it is the zinc corroding, rather than the steel.
Noted by the dealer warranty manager:
Zinc inclusion spots to several areas of the tail gate
Car is well presented with no signs of driver abuse or damage
Paint depths show car has no signs of previous repair or replacement panels
I have met all criteria for my side of the warranty contract, but VW will not honour theirs.
VW, please do the right thing, and contact me to resolve.