Hi Juoolz,
This does seem a little odd and I’ve not been able to reproduce this on my end. I took and made three separate meshes for my test case in 4.4.3.
1 - Cube with 8 smoothing groups 1 for each face
2 - Cube with 7 smoothing groups with the front and top in the same smoothing group
3 - Cube with 1 smoothing group with all faces in the single smoothing group
My result was as expected with 24 vertices and 12 triangles.
I only had a single UV.
Here is my test FBX with all three cubes.
Can you test this and see if you’re getting the same results? Can you also tell me which version of the engine are you using currently?
Thank you!
Tim