For some reason, GetPropertyChangeSignal:Connect doesn't work. Why?

Im writing a script for ghost, using Humanoid.HipHeight, so it can "fly" above other creatures, but without changing height (Position Y) of the ghost, based on creatures it flying above . Here it is:

--// Humanoid of Ghost
local Humanoid = script.Parent.Humanoid

--// The part, which is base for counting height
local Floor = workspace.Arena.Floor

--// Humanoid root part as it is
local HRPart = script.Parent.HumanoidRootPart

--// The position Y, ghost.HumanoidRootPart should always be (using Floor.Size.X is because Floor shape is a Cylinder. 10 is because base HipHeight of ghost is 10)
local BaseY = Floor.Position.Y + Floor.Size.X/2 + 10 + HRPart.Y/2

--// Floor Y is 0, SizeX is 2, HRPart is standart: 2,2,1. So 0+1+10+1 = (12), this is exactly printed, so it's right.
print(BaseY)

--// Ghost have a boolean value "OnArena" inside, that changes once it teleported to arena.
script.Parent.OnArena.Changed:Wait()

--// Me checking if "OnArena" accepted and script continue.
print("ON ARENA")


--// THIS is never works.
HRPart:GetPropertyChangedSignal("Position"):Connect(function()
	--// Never prints :)
	print("It does change the position, at least cuz its walking lol")
	if HRPart.Position.Y ~= BaseY then
		Humanoid.HipHeight = Humanoid.HipHeight - (HRPart.Position.Y - BaseY)
	end
end)

No errors in console. So, i have no idea why moving Ghost doesn't trigger change of it's HumanoidRootPart Position.

GetPropertyChangedSignal("Position") and some other properties (like CFrame) don't fire at all. Not sure why, but I think it's intended behaviour.