Player characters appear to "hang"/"freeze" in place on other players' clients when tweening the size of a part welded to the character

[CatzRule_81 (Catz)]

The problem:

Whenever tweening a part on the server that is welded to a player’s character, that player’s character appears to “hang” wherever it is on other clients’ screens until the tween is complete. This can cause major issues with other scripts that detect hits based on where the server thinks the parts are, when on another client’s screen (the person with the part welded to their character) it may be in an entirely different spot.

I’ve done some digging and I believe I’ve traced the issue to an unintended failure in client-server replication. When a part welded to a player’s character has its size tweened, the server fails to properly process the movement of the player’s character. This issue is then replicated to all other clients, creating the “hanging” or “freezing” effect.

Reproduction steps:

1. Either create a local test server in Studio with 2 players or use a live Roblox server with 2 players.
2. Weld a part to any part of a player’s character that is unanchored, non-collideable, and massless. Rig type of the character doesn’t matter.
3. Tween the size property of the welded part. In all tests I have conducted, tweening other attributes such as transparency or color does not create the same effect.
4. On the client of the player who has the part welded to them, everything should appear normal. However, on other players clients, the character of the player with the part welded to their character should appear to “freeze” wherever they are until the tween completes.

Additional information:

This issue has been documented to an extent by another DevForum user. Seen in this post is a video of what occurs when tweening a character-welded part from the server side in addition to a significant amount of other relevant information.

How do I prevent freezing the my Rig on other Clients when Welding a part to my Character?

When I Weld a part to my character and play an animation, it works fine on my client but not others. How the script works is the animation is played on my end, an event is fired to the server to then tell the clients to replicate it on their end. All the clients run the same script each with the original client’s character as a parameter. The Part used for animation is Massless, Unanchored, and CanCollide is turned off. It welded to the Character’s Right Arm and I have tried using a Weld, WeldC…

In all tests I have conducted, I have used an R6 character with no additional accessories, and the part I have tweened was welded to the character’s HumanoidRootPart. However, the issue should be reproduceable by welding the part to any part of the character, so long as it is unanchored/non-collide/massless as mentioned earlier.

Intended behavior:

All players’ clients should see the same thing.

Actual behavior:

As mentioned earlier, the server fails to process the character movement of the player with the welded part during the tween, causing it to erroneously replicate this to all other clients.

Workaround:

Tweening the scale of a SpecialMesh applied to the welded part works fine with no issues.
EDIT: @ContextLost also pointed out changing the RootPriority of the HumanoidRootPart to something >0 is another viable workaround, thanks for pointing this out

Apologies if this is badly worded, I have Autism so I have issues communicating with others. If you need anything else please do let me know and I’ll see what I can do. Thanks for any help anyone can provide

[V4_Ap3Dud3]

Hello and thank you for the report, our team will investigate. In order to aide in our investigation, can you attach a sample rbxl file that reproduces the issue?

[ContextLost]

Take a look at AssemblyRootPart and see if it’s consistent on the client and server at all times. It’s probably not.

Size is one of the properties considered for root priority. See: Understanding Assemblies | Documentation - Roblox Creator Hub

AssemblyRootPart is the network identity of the assembly for physics replication. Network ownership is implemented as an internal hidden property on the root part instance and that part’s internal instance network GUID is used as the network ID of the rigid body for physics network sync.

Also when the root changes on the server, it can sometimes take a few frames for the server to re-assign ownership of the “new” assembly back to the client and for the client to pick up that assignment and start simulating again. It’s generally beneficial to make sure the AssemblyRootPart of the character does not change if you can avoid it at all.

You can try setting RootPriority on the HumanoidRootPart to >0 so it will always take priority even over a very large unanchored part in the same assembly.

[CatzRule_81 (Catz)]

@ContextLost

Take a look at AssemblyRootPart and see if it’s consistent on the client and server at all times. It’s probably not.

Just checked, the assembly root doesn’t change at any time on the character’s model. Thanks for bringing up this property though, I’ll see if this is a viable workaround for the problem and update my original post if it’s functional.

@V4_Ap3Dud3

In order to aide in our investigation, can you attach a sample rbxl file that reproduces the issue?

tweenservice root weld bug.rbxl (55.1 KB)
EDIT: Accidentally forgot to remove a few empty folders that my plugins generated, shouldn’t be a problem since this bug occurs both in studio and on live servers.