Normal issue upon importing mesh

Hi everyone, new user here.
Hope this wasn’t discussed previously, I just imported a mesh from Blender I modeled, everything is fine there, no issues, but as soon as I import it in Instamat as a layering project I get these weird gradients showing in the shading. It should all be flat, like a dark grey in this particular case. I did a raytracing render and as you can see, this is a “normal shading” issue, because it doesnt display on any other AOV.
Any idea where it might come from and on how to fix that ?
Thanks in advance