This is actively affecting our game, Carry an Egg, and is preventing us from developing important upcoming updates. We have lost a significant amount of CCU due to this bug.
Essentially, when a player has a container in front of them (eg. a box) welded to their HumanoidRootPart with an unanchored part in it (no matter what shape), jumping will cause the player to fling in the opposite direction at fast speeds.
Here is a video reproducing the bug in a separate place with bare-bones reproduction steps:
Here is a place file where the bug can be tested.
FlingBugReproduction.rbxl (68.5 KB)
This is a recent issue and is not caused by any updates created by us. Our last published version was on August 6th, and I have tested this on a version published on August 3rd where the same issue occurs in Roblox Studio (both versions were known to be very stable). We first observed this problem on September 3rd, around 21:00 UTC.
Expected behavior
As was the behavior before, I expect the player to be able to jump with no problem, and for the unanchored part to bounce around in the box without affecting the player’s motion this significantly.