UInstancedStaticMeshComponent::AddInstance spawns in world space

I have actor with InstanceStaticMesh component, positioned at [0, 900, 0]. In it’s begin play, I then spawn mesh instances like this:

FTransform t;
t.AddToTranslation(FVector::ForwardVector * 4 * (meshDims.X + interval));

the X of the translation added is computed to be 440. I expect the function to add the instance in the local space of my instancedMesh actor - so it’ll be [440; 900; 0]

Instead, it is treated as world space coordinates and my mesh (cube) is added to [440; 0; 0];
Using AddInstanceWorldSpace does the same.

What am I missing here?