I'm using an UV map node for some cycles materials, both being generated by a script of mine. Everything is OK in the viewport and in renders but in the material preview tab it appears with an uniform color corresponding to the right border of the texture. As soon as I disconnect the UV map node from the diffuse texture, the preview is OK again. If I reconnect it the preview shows again an uniform color. It this so for the Diffuse shader or the diffuse entry in the principled shader.
If I play with the UV mapping, either with the mapping node or the mapping in the Properties -> Texture tab, it seems that I can change the position but in the preview pane any change in the size seems to not have any effect.
I suspect UV mapping used for the viewport & render are not the same than for material preview.
So the question is "why the preview show only the color on the border of the texture while what can be seen in the viewport or in renders is OK since it corespond well to the texture and the UV map"?