BillboardGui disappearing after decreasing to a certain size?

Hey guys, so I was creating a custom health bar over the player's head using BillboardGui and it works fine except when the size of the bar falls to ~37% of its original size it just vanishes! If you could help me, that'd be awesome. Here's the script I'm using:

local gui = Instance.new('BillboardGui', script)
gui.Size = UDim2.new(1,0,1,0)
gui.StudsOffset = Vector3.new(0,3,0)
local l = Instance.new('Frame', gui)
l.Position = UDim2.new(-1.5,0,0,0)
l.Size = UDim2.new(4,0,.5,0)
l.BorderSizePixel = 0
l.BackgroundTransparency = 1
local h = Instance.new('Frame', l)
h.Size = UDim2.new(1,0,1,0)
h.BorderSizePixel = 0
h.BackgroundTransparency = .6
h.ZIndex = 2
h.BackgroundColor3 = Color3.new(0,.8,.15)
local t = Instance.new('TextLabel', h)
t.Size = UDim2.new(1,0,1,0)
t.TextColor3 = Color3.new(1,1,1)
t.ZIndex = 3
t.BackgroundTransparency = 1
t.Text = '100%'

game.Players.PlayerAdded:connect(function(plr)
plr.CharacterAdded:connect(function(chr)
if chr:findFirstChild("Head") then
local humanoid = chr:WaitForChild('Humanoid')
local temp = gui:clone()
temp.Parent = chr.Head
humanoid.HealthChanged:connect(function(health)
    local healthPercentage = health / chr.Humanoid.MaxHealth
        temp.Frame.Frame:TweenSize(UDim2.new(healthPercentage,0,1,0),"Out","Bounce",0.7,true)
        temp.Frame.Frame.TextLabel.Text = (math.floor(healthPercentage * 100) .. "%")
            if health >= 70 then
            temp.Frame.Frame.BackgroundColor3 = Color3.new(0,.8,.15)
            else
                temp.Frame.Frame.BackgroundColor3 = Color3.new(.88,.72,0)
                wait()
            end
        end)
end
end)
end)

Also an additional question, how would I make it so the bar also goes to red if the player's health falls below 30%? I can't do it without something going wrong.