Well… That is the old native .babylon file version. I have been trying to get out the newer toolkit. which is GLTF based… So basically EVERYTHING to do with the render of a scene has changed… its gltf based (beautifully PBR Rendered)
I cant tell what wrong with your project (It may be the importMesh call vs a scene load).
FYI… I dont have the Particle System class anymore in the new version… You would need to create a script component and create your particle system using code in your script component. (I may look at making another attempt at creating a Unity Style Shuriken particle System again for the new babylon toolkit)
Can you package up the old project and send to me and i will take a look at it so see if i can tell what is going on
But for now… I would recommend starting to use script components and create your particle system that way… When the new toolkit comes out… it will much easier to port your existing project content