Relative physics desync with ImprovedPhysicsReplication enabled

Bug Reports Engine Bugs
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1

I have an unanchored part on a baseplate.

I have 2 scripts. A client script that sends a remote to the server when I hit E, and a server script that anchors the character root when it receives the remote event then shortly unanchors the root part.

Server script

local players = game:GetService("Players")
local replicatedStorage = game:GetService("ReplicatedStorage")
local runService = game:GetService("RunService")

local ANCHOR_DURATION = 5.2
local TRACE_WINDOW = 8
local TRACE_THRESHOLD = 0.5

local remote = Instance.new("RemoteEvent")
remote.Name = "AnchorTest"
remote.Parent = replicatedStorage

local busy = {}


remote.OnServerEvent:Connect(function(player)
	if busy[player] then return end

	local character = player.Character
	local root = character and character:FindFirstChild("HumanoidRootPart")
	local humanoid = character and character:FindFirstChildWhichIsA("Humanoid")
	if not root then return end

	busy[player] = true

	-- chad reaches UseAbility through a task.spawn off the remote handler
	task.spawn(function()
		
		root.Anchored = true

		task.delay(ANCHOR_DURATION, function()
			busy[player] = nil
			root.Anchored = false
		end)
	end)
end)

players.PlayerRemoving:Connect(function(player)
	busy[player] = nil
end)

Client script

local userInputService = game:GetService("UserInputService")
local replicatedStorage = game:GetService("ReplicatedStorage")

local remote = replicatedStorage:WaitForChild("AnchorTest")

userInputService.InputBegan:Connect(function(input, gameProcessed)
	if gameProcessed then return end
	if input.KeyCode ~= Enum.KeyCode.E then return end
	remote:FireServer()
end)

When you jump off the unanchored part then hit e, you character anchors in mid air. On unanchored, the players character movements are then transposed on the server until they jump back on the unanchored part.

Video:

Replication place attached. Replication steps.
1.) Jump on the part.
2.) Jump off the part, hit e mid air.

test.rbxl (62.8 KB)

Disabling ImprovedPhysicsReplication prevents this issue.

Expected behavior

The client and servers view of the players character should remain synced after unanchoring.

This undesirable behavior was discovered in our game Yeet Battles after some debugging when certain abilities resulted in users characters going off to Narnia :sob:.

2

I would recommend reporting this here: Upcoming Improvements to Physics Replication

Staff are monitoring the thread and acknowledging bug reports, at least

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