Part drifting sideways for an upward force

[Pifinity3 (Pifinity)]

I have a Part in Workspace containing children Attachment and VectorForce. The Part’s network owner is the player. I initially wanted the Part to hover. I wrote a quick script similar to:

-- OBJ VARIABLES
local part = game.Workspace.Part
local vectorForce = part.VectorForce

game:GetService("RunService").PostSimulation:Connect(function(dT)
	vectorForce.Force = Vector3.new(0, part.AssemblyMass * workspace.Gravity, 0) 
end)

The Part hovers, but it has a velocity sideways. What is the force that causes the Part to act like so? How can I prevent or cancel this force?

[Pifinity3 (Pifinity)]

Note that this is a basic setup but is part of a larger system.

[Pifinity3 (Pifinity)]

I used a different body mover, but you can use a PID controller.