How to get SubtractAsync to fully slice off piece of the part

I'm trying to slice a part of this part, so that it matches the bottom part, and the method I'm trying is SubtractAsync.

When I do it however, the part gets sliced, but not so that the remaining part gets removed. Here's what I mean:

How do I edit my code to slice off the piece of the part?

My code:

local Brick = workspace.Brick
local Stack = Brick:Clone()
local TS = game:GetService("TweenService")
local tweenInfo = TweenInfo.new(3, Enum.EasingStyle.Quad, Enum.EasingDirection.InOut, math.huge, true)
Stack.Position = Vector3.new(Brick.Position.X, Brick.Position.Y + Brick.Size.Y, Brick.Position.Z - 55)
local tween = TS:Create(Stack, tweenInfo, {Position = Vector3.new(Stack.Position.X, Stack.Position.Y, Stack.Position.Z + (55 * 2))})
Stack.Parent = workspace
tween:Play()
game.ReplicatedStorage.PlaceDown.OnServerEvent:Connect(function()
    tween:Pause()
    local PartA = Instance.new("Part")
    local PartB = Instance.new("Part")
    PartA.Transparency = 1
    PartB.Transparency = 1
    PartA.Parent = workspace
    PartB.Parent = workspace
    PartA.Orientation = Vector3.new(-90, 0, 0)
    PartB.Orientation = Vector3.new(-90, 0, 0)
    PartA.Size = Vector3.new(15, 0.051, 15)
    PartB.Size = Vector3.new(15, 0.051, 15)
    PartA.Position = Vector3.new(Brick.Position.X, Brick.Position.Y + 2.5, Brick.Position.Z - (Brick.Size.Z/2))
    PartB.Position = Vector3.new(Brick.Position.X, Brick.Position.Y + 2.5, Brick.Position.Z + (Brick.Size.Z/2))
    PartA.Anchored = true
    PartB.Anchored = true
    local SlicedStack = Stack:SubtractAsync({PartA, PartB})
    SlicedStack.Position = Stack.Position
    SlicedStack.Parent = workspace
    Stack:Destroy()
    PartA:Destroy()
    PartB:Destroy()
end)