The part that’s rusting is actually the centre part of the brake disc that bolts to the hub on the car, rather than the hub itself, and it’s very common on the mk7 and mk7.5 Golf, which presumably uses the same brake components as the Leon.
Brake discs are usually made from cast iron and if it’s unprotected, it will rust as soon as it’s exposed to moisture - seen on the braking surface of the discs when they get wet from washing the car. Like you, I also use a pressure washer when cleaning my VW but I don’t use it on the wheels, and the centre (hub) sections of my discs are still pretty much corrosion free after nearly 4.5 years.
The centre section of the disc that you can see rusting only gets a light blowing over of paint, hence why they do often show signs of rusting fairly early on. They are easy to repaint - many Golf owners use smooth Hammerite or a high temperature paint - either black or silver. Remove wheel and support car safely and securely - ideally on axle stands; not the ‘widow maker’ jack if your car has one. Rub down the part to be painted to remove any rust, mask up any parts not to be painted and apply a couple of coats of paint to the prepared surface. They should then be OK for a good 2-3 years and easy to re-do if necessary in the future.