Why does this fall damage script not work when sitting at all?

local damageHeight = 20 -- The height at which the player will start getting damaged at
local lethalHeight = 45 -- The height at which the player will get killed

game:GetService("Players").PlayerAdded:Connect(function (plr)
	plr.CharacterAdded:Connect(function (char)
		
		local root = char:WaitForChild("HumanoidRootPart")
		local humanoid = char:WaitForChild("Humanoid")
		
		if humanoid and root then
			
			local headHeight
			
			wait(3) -- Prevent the player from dying on spawn
			humanoid.FreeFalling:Connect(function (state)
				if state then
					headHeight = root.Position.Y
				elseif not state and headHeight ~= nil then
					pcall(function ()
						
						local fell = headHeight - root.Position.Y
						
						if fell >= lethalHeight then
							humanoid.Health = 0
						elseif fell >= damageHeight then
							humanoid.Health = humanoid.Health - math.floor(fell)
						end
					end)
				end
			end)
		end
	end)
end)

Here is the whole script, after a couple of months working on a game I have no idea why it doesn't work. Could somebody help me? I've asked my friend and he doesn't know why it doesn't work with sitting.