Newly uploaded animations work differently than old ones

Edit: I noticed this seems to be related to AnimationWeightedBlendFix. Disabling that fixes it, but this seems to be a bug in the new animation engine.

I noticed something strange in my game: the old uploaded animations work fine for my character, but if I download this old animation and re-upload it as a new one it breaks.

It is as if something changed some time ago that causes the new animations to be uploaded with different data which causes twitchy playback and missing frames.

I wasn’t able to find anything on the forum, does anyone know what happened?

I also made sure that all keyframes exist but it still doesn’t help.

Somehow the newly uploaded animations (which have an Action priority) do not correctly override Core animation priority.

Here is how it looks:

  1. When the tool at slot 1 is equipped the shoulders/arm do not rotate with the torso (the weapon
    does NOT aim up/down in the camera direction, it always aims straight forward).
  2. When the tool at slot 2 is equipped the shoulders/arm DO rotate with the torso correctly.
    The weapon DOES aim up/down based on the camera pitch.

Why does 1) happen? Why does roblox animation system not respect proper parent-child animation relationship?