Bake To Control Rig produces empty keys when rig uses Import Skeleton in Construction Event and binding is a spawnable in a subsequence

Bake To Control Rig fails to key procedurally-created controls when all three of the following are true:

1. The Control Rig creates its hierarchy at runtime via Import Skeleton in the Construction Event (bones and controls do not exist in the asset's stored initial hierarchy)
2. The character is a spawnable whose binding lives in a subsequence
3. Evaluate Sub Sequences In Isolation is OFF

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Changing any one of these three conditions makes the bake succeed. This points at the rig instance used by the bake being constructed/evaluated without the correct binding context (or with an unapplied sub-section time transform) when evaluation goes through the root sequence hierarchy.

Expected

Bake To Control Rig should construct/evaluate the rig instance with the resolved spawnable binding and correct time-space regardless of where the binding lives in the sequence hierarchy — or at minimum surface an error instead of silently producing an empty bake.

Why it matters

Construction-time hierarchy import is the foundation of modular/procedural rigging, and spawnables-in-subsequences is the standard shot structure for linear content pipelines. The intersection silently breaks bake-to-rig, and the failure looks like a rig authoring error to end users (partial keys, no message), which makes it expensive for studios to diagnose — this one cost a client several days.

Workarounds (validated)

Enable "Evaluate Sub Sequences In Isolation" before baking; or bake in a root-level duplicate of the sub; or use a rig variant with the constructed hierarchy stored in the asset.

Minimal repro assets (stock content only)

• CR_Repro_ProcBake: Control Rig on SK_Mannequin. Construction Event: Import Skeleton → For Each bone → Spawn Transform Control (name derived from bone name at runtime, no literal name pins). Forward Solve: controls → Set Bone Transform. Backwards Solve: Get Bone Transform → controls. ~8 nodes, 2 bones is sufficient.
• Root LevelSequence with playback range starting at a non-zero frame (e.g. 1001)
• Char subsequence containing Manny as a spawnable, with a stock AnimSequence on its animation track

Repro steps

1. Open the root sequence, dive into the char subsequence
2. Confirm "Evaluate Sub Sequences In Isolation" is OFF
3. Right-click the skeletal mesh track → Bake To Control Rig → CR_Repro_ProcBake
4. Inspect the resulting Control Rig section

Result: controls receive no keys (or only statically-authored controls receive keys; procedurally spawned ones are blank). No error is surfaced to the user; the bake appears to complete.

Results matrix (single variable changed per row, same assets/session):

Row 2 is the key repro: identical assets, one editor checkbox toggles pass/fail.

Log evidence (attached, two sessions)

• Session A (production rig, loud failure): during bake evaluation, every FRigUnit_GetTransform reports Bone(...) is not valid — the rig instance evaluated by the baker has an empty skeleton, i.e., Construction ran with no skeletal mesh available to Import Skeleton. Procedurally spawned controls therefore don't exist, followed by Ensure: Channels.IsValidIndex(ChannelIndex) in MovieSceneChannelHandle.cpp (track channel list built from a different control set than the evaluated instance).
• Session B (silent failure): working and failing bakes 4 minutes apart in one session, same assets, only the nesting changed. The failing bake logs zero errors and produces a transient AnimSequence nearly identical in size to the working bake's (1.92 vs 1.93 MB) — source animation sampling and rig construction succeeded; keys are lost downstream in the key-writing path.