I am not sure if this is a Roblox bug or not. It seems that if you create frame guis that are the size of a pixel some tend to be missing.
on the left is the Roblox version and on the right is the c++ version (.tga img glfw opengl ndc texture for viewer)
these are the exact obj model, the code is pretty much the same for both of them except one is written in Lua and one is written in c++ but they all follow the same pseudo code if you will. this could probably be an error on my end, maybe floating point errors (c++ one is based on an all int based model) but I am using math.round() to combat that and my bresenham one (this isn’t scanline) shows some of these artifacts too