Well, Pedro, waddya know. Being a CGI man myself, I was sure it was a phong / softness thingy, but raising the texture quality to maximum fixed it. It's another kind of rendering, obviously, so I'll throw an egg on mi face.
With Vulkan off, setting texture to max doesn't fix it, though. It only works with Vulkan. If I can't fly with textures on max, I'll complain again :-)
On X-Plane 10 this doesn't happen (and I must confess I still like XP10 a lot).
Ok, thanks guys for the replies. It's not a big deal, really.