Mystery solved: the fan was assembled backwards, apparently from the factory.
The fan is actually a "hamster wheel" that spins and throws air outwards. But because it has profiled blades, it can actually suck air inwards when spinning the wrong way round.
Anyway, quick notes. Do not try do disassemble the fan to clean it, as the plastic casing will start crumbling to pieces when you unscrew it from the metal bracket. The actual "hamster wheel" can be removed without much trouble: just remove the fan and metal bracket assembly from the car, remove the inlet shroud of the fan, remove the spring clip that holds the fan and pull the fan using an Allen key trough one of the fan's holes. There's no poka-yoka to prevent the fan from being assembled the wrong way round, but one side of the fan's hub is longer than the other. Short side is outwards.