I don't think that this is a bug.
You defined the div of the "li" element with 15vw.
The container has a width of 50vw.
1 VW is 1/100 of the whole Viewport.
So what you get is a container with half of the viewport, in which are 4 divs, which take 60% of the viewport.
There are 10% more than the container even have plus the spacings between them. I don't know if I misunderstood something, but in my opinion the width are just false which you set?