UE 5.8.4 Per-bone Blend Space sample smoothing overriding is skipped in packaged builds

General Issues and Bug Reporting (Unreal Engine)
1

Summary

Per-bone sample smoothing on Blend Spaces appears to be skipped in packaged UE 5.8.4 builds. The same setup smooths in the editor, but in a packaged Win64 build the blend changes instantly as if the per-bone smoothing overrides were off.

What Type of Bug are you experiencing?

Animation

Steps to Reproduce

  1. Create a Blend Space with at least two animation samples.
  2. Enable per-bone sample smoothing and set a slow interpolation speed for a bone chain.
  3. Play it through a simple Animation Blueprint and change the blend input from one sample to another.
  4. Compare editor playback with a packaged build.

Expected Result

The packaged build applies the authored per-bone interpolation speeds, as the editor does.

Observed Result

The affected bones switch immediately in the packaged build. Setting a positive global Weight Speed makes the blend smoother, but does not restore the per-bone overrides.

Affects Versions

5.8

Platform(s)

Windows

Additional Notes

In UE 5.8.4, UBlendSpace::PostLoad() calls InitializePerBoneBlend() inside #if WITH_EDITOR. PerBoneBlendValues, which that function fills, is not a serialized UPROPERTY. In UE 5.7, the call is after the editor-only block. This looks like a possible cause; I have not instrumented the packaged runtime object to confirm its cache contents.