enabling Nanite on merged actors crashes Unreal (5.3.2)

Hi everyone !
I have a building made of many static mesh actors
When Nanite is enabled on each individual asset/actor , it works fine.
BUT when I merge all my actors into one and enable Nanite on the merged asset, it crashes my session.

I thought that it was because of the material slots limitation, but although the number of mesh is large, i only have ten different materials.

Using Unreal 5.3.2

Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Public\Containers\Array.h] [Line: 771] Array index out of bounds: 16777215 from an array of size 64
UnrealEditor_NaniteBuilder
UnrealEditor_NaniteBuilder
UnrealEditor_MeshBuilder
UnrealEditor_MeshBuilder