Steps to reproduce:
1. Open the “user’s attached project”
2. Open the “Demo 3D Open World” Scene
3. Enter Play Mode
4. Use “wasd” or arrow keys to trigger an Animation Transition
5. Observe the crash

Alternatively:
1. Open the “user’s attached project”
2. Open the “Assets/Toon Boom Harmony Gaming SDK/Samples/Demo 3D Open World/Art/SLY_Character Animator Controller.controller”
3. Through the Animator, select the “Sly_Fox-P_sly_idle” → “Sly_Fox-P_sly_run” transition
4. Observe the crash

Reproducible with versions: 6000.2.0b2, 6000.2.6f1, 6000.3.0b3, 6000.4.a1
Not reproducible with versions: 6000.0.58f1, 6000.1.16f1
Couldn’t test with versions: 6000.2.0a1, 6000.2.0b1 (due to animation controller and the Animation Controller breaking)

Reproducible on: macOS 15.6.1 (M1 Max), Windows 11 (user’s machine)
Not reproducible on: no other environment tested

Note: Also reproducible in Standalone Player

First few lines of StackTraces:
{{#0 0x0000010540eb84 in void mecanim::ValueArrayCopy<false>}}
{{#1 0x000001053e286c in AnimationLayerMixerPlayable::LayerMixerProcess}}
{{#2 0x000001053e2974 in AnimationLayerMixerPlayable::ProcessAnimation}}
{{#3 0x000001053dafac in AnimatorControllerPlayable::ProcessAnimation}}
{{#4 0x00000105351d50 in (anonymous namespace)::ProcessPlayableGraph}}

0x00007FFB89EEE4A8 (Unity) mecanim::ValueArrayCopy<0> 
0x00007FFB89EC4BC7 (Unity) AnimationLayerMixerPlayable::ProcessAnimationSingleLayer 
0x00007FFB89EC3C25 (Unity) AnimationLayerMixerPlayable::LayerMixerProcess 
0x00007FFB89EC48A0 (Unity) AnimationLayerMixerPlayable::ProcessAnimation 
{{0x00007FFB89EBAA98 (Unity) AnimatorControllerPlayable::ProcessAnimation}}