Spamming AnimationTrack:AdjustWeight(0) positions body parts to NaN

Bug Reports Engine Bugs
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1

This is a very strange bug.

Basically, if you have an AnimationTrack with a weight greater than 0 playing on an R15 character, and then you start calling AnimationTrack:AdjustWeight(0) every frame, then after some time, all of the character’s body parts will suddenly have their Position set to [nan, nan, nan].

This only seems to happen on mobile devices. I’m not able to repro on desktop.

Additionally, spamming AnimationTrack:AdjustWeight(0) causes AnimationTrack.WeightCurrent to remain at a seemingly random number above 0 indefinitely, even though the animation does not visually appear on the character anymore. And yes, I know that WeightCurrent will typically always be off from WeightTarget by a very small amount, but this is not a small amount… WeightCurrent can typically be anywhere from 1 to 10, while WeightTarget will remain at 0.

Repro

  1. Start playing an animation on an R15 character. Make sure that the weight is greater than 0.

  2. Start calling AnimationTrack:AdjustWeight(0) every frame.

  3. Note the large difference between WeightCurrent and WeightTarget.

  4. After a few seconds of this… your character will vanish!

Demonstration

Here is a simple repro place showcasing this bug:
Animation weight bug.rbxl (71.6 KB)

Since this bug only seems to happen on mobile devices, you should publish this place to Roblox and join on a phone, instead of just playtesting in Studio.

After you join, an animation will automatically play on your character, AnimationTrack:AdjustWeight(0) will start being called every frame, and WeightCurrent, WeightTarget, and Character.Head.Position will be recorded in a GUI.

Workaround

If you need to update animation weight every frame, you should check to make sure the new value wasn’t already set, for example:

local newWeight = 0

if (animationTrack.WeightTarget ~= newWeight) then
    animationTrack:AdjustWeight(newWeight)
end

Device information

  • iPhone SE 3
  • iOS 26.2
  • Roblox for iOS, version 2.702.633

Expected behavior

Calling AnimationTrack:AdjustWeight(0) shouldn’t cause body parts to be positioned to [nan, nan, nan].

2

Hi, thanks for your report. Are you enrolled in any specific betas ?

3

No, at least not that I know of. I’ve never opted into any Early Access Programs.

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