From what you say Mk7 owners are safe to assume that these vents are an essential component in the Golf design and as such all Golf models should be fitted with something similar.
If that is correct perhaps then all Golf owners should consider those vents on their vehicles a potential or present water ingress problem?
VW as a brand is on a cliff edge as far as I am concerned
These vents are on all cars behind the bumpers, its a very common way to expel air in the said situations. Google 'A class water in boot', or 'Ford Focus water in boot'. You will see many owners having issues and solving them by either buying new vents with a new seal as they aren't often available separately or tiger seal-ing them into position. Most common reason is the foam seal on the edge of the vent either deteriorates over time, or a VW worker 5 years ago on a production line creased the seal a small amount when fitting. Otherwise they would all likely fail at the same time in millions of Golfs, Fords, Vauxhalls, Mercs, BMW etc, which they haven't, at least in the case of the VW and Golf model.
The other potential reason could be a small but significant crash that results in the repair panel and vent not fitting perfectly, possibly due to an aftermarket panel, but you have had your car from new so I assume that's unlikely.
Either way if this repair was carried out by a main dealer at your request then that is a reason for concern, but I wouldn't be too worried about VW as a brand, everyone is cutting corners these days, just look at the Mk8 threads! I can't think of a car I have own in the last 10 years that hasn't had cost cutting issues, actually I think Toyota seem pretty good but most of the German brands are average at best.
hope you get it sorted with VW.
