Why is my placement system's offsets breaking when trying to place on a rotated surface?

I am making a placement system for my game but when I try to place blocks that are rotated (not axis-alinged) the offsets break.

local function GetPlacementPosition(gridSize, partSize, position, normal, target)


    local gridCorner = Vector3.new(
        math.floor(position.X / gridSize + 0.5) * gridSize,
        math.floor(position.Y / gridSize + 0.5) * gridSize,
        math.floor(position.Z / gridSize + 0.5) * gridSize
    )
    return gridCorner + partSize * normal  / 2 
end

local pos = GetPlacementPosition(grid.Value, preview.Parts.Hitbox.Size, target, ray.Normal, plrMouse.Target)

local cf = CFrame.new(clamp pos to plot walls)

preview:SetPrimaryPartCFrame(cf)

Rotated 90 deg: prnt.sc/131hf0l

Rotated 45 deg: prnt.sc/131hdtp

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