I donĀ“t know, but it should be, because I ported the model from fs15 and assigned textures. In fs15 it works correctly.
I've checked, the UV are wrong in the model itself. So, it's either you are using the wrong decal texture, or FS15 does some weird trickery with the decals.
It is possible to fix it by moving the nodes to the right place in the uv/image editor? --- Post updated --- Yes, it works.