WoWInterface - Irregular / Missing border

Hi,

I have 5 x 10 frames. On each frame I do the following:

Lua Code:

local backdrop = {
    bgFile = LSM:Fetch("background", "solid"),
    edgeFile = LSM:Fetch("border", "solid"),
    tile = false,
    tileSize = 0,
    edgeSize = 1,
    insets = {
        left = 0,
        right = 0,
        top = 0,
        bottom = 0
    }
}
 
frame:SetBackdrop(backdrop)

The Problem: The first frame in each of the 5 rows misses the right border like shown in this image:

All other frames are showing the borders correctly.

What is the problem here?

Some more specific stuff:

When I say the border is 1, it is not really 1. It is calculated like the following:

Lua Code:

local bestPixelSize = max(0.4, min(1.15, MATH:Round((768 / SCREEN.info.height), 5)))
local pixel = 1;
local ratio = (768 / SCREEN.info.height);
local border = ((pixel / bestPixelSize) - ((pixel - ratio) / bestPixelSize));