That's a black surface mould, that's very common and more often found on stored timber ordecking. As there is no antifungal treatment applied, and the thermal treatment process has no effect at all on the surface resistance to mould growth like this, then this is exactly what I'd expect to see on a timber like ash that has no natural antifungal resins etc that would tend to prevent it.
The mould not going cause rot in the timber, but will stay on the surface, because the thermal treatment makes the internal structure of the timber resistant to the growth of mould beyond the surface. It will tend to proliferate on any surface that stays slightly wetter, or cooler, than any another, and the change in grain structure between adjacent panels will be more than enough to cause the surface moisture or temperature difference needed in order to cause the effect you're seeing. The black mould species is probably very similar to the types found growing on cool, damp, bathroom surfaces, and the patterning where the battens are is most probably due to slight surface temperature variations in those areas
It should wash off fairly easily, but may leave some staining, and will be a recurring problem, as thermally treated non-durable timber has no inherent resistance to surface mould growth at all, it needs some form of surface treatment to prevent this.
I think you may have been mislead as to the properties of this material, most probably by a sales person. There's nothing that I can find anywhere in the company literature that suggests there is any resistance to surface mould growth, but there do briefly mention that customers may wish to consider some form of surface treatment or stain, which sort of skates around the problem that they know that surface mould may be an issue in some climates.
Ash is a non-durable timber, with virtually no inherent resistance to rot. In it's untreated form it's durability rating is 5, as poor as you can get, about the same as balsa wood; many softwoods are a great deal more durable. The thermal treatment improves the durability rating to 1, but has no effect at all on the surface properties. There are no resins or mould-resistant compounds within ash, so even thermally treated it is still going to attract surface mould growth, much the same as the way softwood decking does.
You can try and fight the supplier, and the advice that @Ferdinand has given is about as good as any I can think of, or you can clean the surface up and treat it with something like Sioox, as already suggested. My personal opinion is that you've been misled as to the difference between durability and surface appearance, but proving that could be a significant challenge.