Does anyone know why the first layer of an object (let’s say a simple plate) over a support, is printed parallel to the support zig-zag and not perpendicular ?
This makes a poor-quality first layer as the filament is not supported for the full length of the zig-zag, while it would be obviously more efficient perpendicular to the zig-zag.
Is this normal ?
Thx
I think it’s a coincidence. You can try rotating the object or enabling support interface and maybe increase the support interface density. Depending on the slicer you’re using, there could also be a separate setting for the orientation of the pattern too…
The slicer is Luban.
Looking at the Luban-embedded help on zig-zag, I find that, which is exactly the contrary of what i’d expected, and makes the bug probably on purpose 
The zigzag pattern is like the lines pattern, but the lines are connected on the ends.
Fairly sturdy, greatly increasing reliability.
Provides the best overhang quality of all patterns together with Lines. The lines are very close together and oriented such that they are not perpendicular to the skin.
Maybe enable support roof and/or support interface then? 
In Cura you can control top/bottom line directions…
I’m guessing you got unlucky on your layer height and support separation distance, and the gap is just right to skip the layer that would normally print perpendicular to the support. Use the preview slider and run it up and down until you find the top layer of the support, then check the next few layers to see what it’s doing. Once you click on the preview slider, the up and down arrow keys go up and down a single layer. The slider is not very precise when dealing with large prints.
You can play with various rotations and parameter settings, and keep checking preview until you get something more sane. I had a part where I was trying to print a single layer bridge, making a feather-ish texture on some wings. I spent a lot of time rotating and adjusting layer parameters to get the texture and print direction how I wanted it. The preview saved me a lot of time.
Thanks a lot gentlemen. This makes sense clearly.
I’ll investigate this and let you know.