Why does mouse.Hit.p give a different value when the camera is manipulated?

I made the following script to create rts style gameplay:

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()
local char = player.Character
local cam = game.Workspace.CurrentCamera

function ShowPointer(camera, target)
    for i,v in pairs(game.Workspace.CurrentCamera:GetChildren()) do
        if v.Name == "Point" then v:Destroy() end
    end
    local point = Instance.new("Part", camera)
    point.Name = "Point"
    point.Size = Vector3.new(1,1,1)
    point.CanCollide = false
    point.Anchored = true
    point.Position = target + Vector3.new(0,1,0)
    point.BrickColor = BrickColor.new(21)
    local mesh = Instance.new("BlockMesh", point)
    mesh.Scale = Vector3.new(0.3,1,0.3)
    char.Humanoid.WalkToPoint = target
    local dist = (char.Torso.Position - point.Position).magnitude
    while point and dist > 4 and wait() do  
        dist = (char.Torso.Position - point.Position).magnitude
    end
    point:Destroy()
end

function OnButton2Up()
    ShowPointer(cam, mouse.Hit.p)
end

mouse.Button2Up:connect(OnButton2Up)

You can just copy and paste this script in a new project and it will work as it should, but whenever I add some camera manipulation, the mouse.Hit.p value messes up.

game:GetService("RunService").RenderStepped:connect(function()
    cam.CoordinateFrame = CFrame.new(char.Torso.Position) 
                        * CFrame.new(0,30,10) 
                        * CFrame.Angles(-20,0,0)
end)

Does anyone know how I can fix this?