PLEASE DO NOT TL;DR THIS IS AN IMPORTANT BUG
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I have finally found a repro for a bug me and a few other people have been encountering for a few months!
When you weld a part, or a model (I haven’t done enough testing to know exactly what as this bug is really hard to reproduce) to a player, or yourself locally sometimes the whole position replication stops working, not just for you but for the whole server.
It is very hard to explain what exactly is going on, but in my case, explained here a couple of months ago Character position stops replicating after welding? I have been making a system where all characters run locally. The server only sees the HumanoidRootPart and each client creates the rest of the model (The arms, legs, head, etc.) and welds it to the server side HumanoidRootPart, but for some reason when this is done the position of the HumanoidRootPart stops replicating completely for both, the player who welded the model and the player who got the model welded to him.
I have spent some time doing research and I managed to find the least amount of code possible to reproduce this bug, although it isn’t as severe where it stops replicating position completely, it still does stop replicating some of the humanoid states, or features like jumping. Here is a video of what I mean:
VIDEO 1
As you can see, like 99% of the jumps which the player on the right did, the player on the left didn’t even see.
Here is what it looks like when it’s more severe and the whole position stops replicating, although I was not able to find a repro for this (I know I can just send the whole game where this happens in as a repro, but I’ll only do that if one of the staff members asks me for it since it also contains a bunch of other code for my game):
VIDEO 2
As you can see the player on the left is moving, but the server on the right can’t even see if the player is moving or not and when the locally welded model is destroyed the position starts replicating again.
Attached is the repro for the half broken replication seen in VIDEO 1, all you have to do is open up the .rblx file, start up a 2 player server and walk around. You will see that some actions do not replicate at all, if you’re lucky it might stop replicating completely for you, but I have yet to encounter that with this specific code.
Replication Bug.rbxl (21.4 KB)