Why does this script play the tween even though this if statement should be false?

Hi. I'm trying to detect when a player's health goes down. When it does, a bar appears with the size of their old health/100. it then tweens away to transparent. This works, but for some reason the tween is not behaving properly. The bar disappears very quickly, even with the extreme values I put in the tweeninfo that tell it to take 6 seconds to disappear.

local player = game.Players.LocalPlayer
local healthgui = script.Parent.Parent.Parent
local bar = script.Parent
local bg = bar.Parent.Background
local tweenservice = game:GetService("TweenService")
local tweeninfo = TweenInfo.new(4, Enum.EasingStyle.Cubic, Enum.EasingDirection.In, 0, false, 2)
local tween = tweenservice:Create(bg, tweeninfo, {Transparency = 1})

player.CharacterAdded:Connect(function(char)
    local hum = char:WaitForChild("Humanoid")

    bar.Size = UDim2.new(1,0,1,0)
    local lasthealth = 100 --reset
    hum.HealthChanged:connect(function(health)
        if health == 100 or health == 0 then
            healthgui.Enabled = false
        else
            bg.Size = UDim2.new((lasthealth/100), 0, 1, 0) --resize to old health
            if health < lasthealth then
                tween:Cancel()
                --update background
                bg.BackgroundTransparency = 0.8
                tween:Play()
                print(lasthealth, health, tween.PlaybackState)
                lasthealth = health
            end
            healthgui.Enabled = true
            bar.Size = UDim2.new(math.clamp(health/100, 0, 1),0,1,0)
            bar.BackgroundColor3 = Color3.fromHSV((math.clamp(health/100,0,1))/3,1,1)
        end
    end)
end)