Material parameters flickering due to UMG animation switching and multi-threaded execution

When continuously transitioning to UMG animations that have a track targeting the same material parameter within the UUserWidget::OnAnimationFinished event, the number of source inputs to UMovieScenePiecewiseDoubleBlenderSystem transitions from 2 → 1 → 0, leaving a blender with no sources. This results in two ApplyScalar tasks coexisting: one via the blender path and one via the non-blended direct path.

When a large number of entities are present, the entity system execution becomes multi-threaded, causing the order of the two ApplyScalar operations to be non-deterministic, which produces the flickering behavior.

Workaround: Destroying the blender in TOverlappingMaterialParameterHandler::UpdateOutput when blending is no longer needed is a workaround from a licensee.

if (bUseBlending)
{
	...
}
#if 0 // original
else if (!Output->OutputEntityID && Inputs.Num() == 1)
{
	Linker->EntityManager.RemoveComponent(Inputs[0], BuiltInComponents->BlendChannelInput);

	Mixin::InitializeSoleInput(Linker, BoundMaterial, ParameterInfo, Inputs[0], Output);
}
#else // work around
else
{
	if (Output->OutputEntityID)
	{
		DestroyOutput(BoundMaterial, ParameterInfo, Output, Aggregate);
	}
 
	if (Inputs.Num() == 1)
	{
		Linker->EntityManager.RemoveComponent(Inputs[0], BuiltInComponents->BlendChannelInput);
		Mixin::InitializeSoleInput(Linker, BoundMaterial, ParameterInfo, Inputs[0], Output);
	}
}
#endif // end work around

1. Open the attached reproduction project
2. Start PIE
3. Confirm that the icons rendered by UMG on screen are not transitioning smoothly
4. Open the console and apply Sequencer.ThreadedEvaluation.EntityThreshold 1024 to run the tasks on single thread (default value is set to 32 in DefaultEngine.ini)
5. Confirm that the icon transitions now work smoothly