Unreal Engine Issues

UE-349951

Child actor components fail to attach in cooked builds when mobility is set to movable and end up at the origin

Child actor components fail to attach in cooked builds when mobility is set to movable and end up at the origin

EPS
Description

Context:
A Child Actor Component allows an actor in Unreal Engine to contain another actor as its child (component).

Problem:
The problem is inconsistent handling of child actor component mobility from Play in Editor to cooked builds where Child actor components fail to attach in cooked builds when mobility is set to movable and end up at the origin.
The licensee indicated proximal cause is failure of the spawned static mesh components to attach to their child actor component parents at runtime in the cooked build with this error from the log: 

"[2025.10.16-16.36.13:594][ 0]PIE: Warning: AttachTo: '/Game/TheOneAndOnlyMap/Generated/CVWO6G0B3SC7ERCKM8BHU7MTL.TheOneAndOnlyMap:PersistentLevel.SM_ChamferCubes_Blueprint_C_UAID_BCFCE7B3ACA2049902_1563135772.StaticMeshActor_UAID_BCFCE7B3ACA2049902_1112414767' is not static , cannot attach '/Game/TheOneAndOnlyMap/Generated/CVWO6G0B3SC7ERCKM8BHU7MTL.TheOneAndOnlyMap:PersistentLevel.StaticMeshActor_UAID_BCFCE7B3ACA2049902_1112414767_GEN_VARIABLE_StaticMeshActor_CAT_0.StaticMeshComponent0' which is static to it. Aborting.
[2025.10.16-16.36.13:595][ 0]PIE: Warning: AttachTo: '/Game/TheOneAndOnlyMap/Generated/CVWO6G0B3SC7ERCKM8BHU7MTL.TheOneAndOnlyMap:PersistentLevel.SM_ChamferCubes_Blueprint_C_UAID_BCFCE7B3ACA2049902_1563135772.StaticMeshActor_UAID_BCFCE7B3ACA2049902_1497762771' is not static , cannot attach '/Game/TheOneAndOnlyMap/Generated/CVWO6G0B3SC7ERCKM8BHU7MTL.TheOneAndOnlyMap:PersistentLevel.StaticMeshActor_UAID_BCFCE7B3ACA2049902_1497762771_GEN_VARIABLE_StaticMeshActor_CAT_0.StaticMeshComponent0' which is static to it. Aborting."

The licensee also indicated that they stepped through the attach sequence in the debugger, and the problem appears to be in UChildActorComponent::OnRegister() - in editor and in PIE, attachment is done by UChildActorComponent::CreateChildActor() which appears to implement an exception to the mobility relationship rules for attachments. Whereas cooked builds follow the core attachment initialization path which has no such exception, and hence the attaches fail.

Steps to Reproduce

(Internal only) See comments

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Backlogged
ComponentUE - Gameplay
Affects Versions5.6
CreatedOct 21, 2025
UpdatedOct 29, 2025
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