Child actor components fail to attach in cooked builds

Development Platform & Builds
, ,
1

See repro steps and attached repro project for basic repro details.

The proximal cause is failure of the spawned static mesh components to attach to their child actor component parents at runtime in the cooked build. Here is the relevant error message 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.

I 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.

2

Steps to Reproduce

  1. In a blank project with Unreal 5.6.1, place *two* static mesh actors at some distance from the origin
  2. In the level editor toolbar dropdown, choose “Convert Selection to Blueprint Class”
  3. Choose the “Child Actors” option and set the base class to simply “Actor”
  4. Close the resulting Blueprint editing window and save everything - note that the converted static mesh actors are still in the same place as before in the editor, at some distance from the origin
  5. Cook and run a build for PC

Observed behavior: the converted static mesh actors are located at the origin

Expected behavior: the converted static mesh actors are in the same location as in the editor, at some distance from the origin

[Image Removed]

3

Hi,

Thank you for the detailed repro steps, repro project, and cause along with your debugging.

I created a bug report for this issue that can be tracked here if/when it’s approved for public visibility: Unreal Engine Issues and Bug Tracker (UE\-349951\).

There is no ETA as priorities for bugs and features can shift at any time.

I did find a similar issue at Unreal Engine Issues and Bug Tracker (UE\-215536), but the small difference was that leaving the root as static, then setting it to movable after placing it in a level did not fix the attach warning.

That being said if the report I put together ends up being a duplicate of this issue, you can track it there.

Regards

4

I’m closing this case, but feel free to respond here if you have follow-up questions.

Thank you!