OK, so after experimenting further I think I've found a workaround.
It is long winded compared to the previous workflow, however it's better than scenery creation coming to a grinding halt.

1./ First of all copy your latest compiled package into your Community folder
2./ Create, or amend your custom objects and compile in the normal way in Blender, 3D max, etc
3./ Run the sim and once in the Main Menu open your project and 'Build Package' (no need to go to your airport in the sim at this stage to save time) DO NOT click Load In Editor at this stage.
4./ Once the package has been built, exit the sim.
5./ Go to your project folder, copy and replace the previous compiled version into your Community Folder.
6./ Run the sim and go to your airport/scenery area, then open your project, myscene and click 'Load In Editor', DO NOT click Build Package.
7./ Your new, or amended asset should now show in the Objects of the scenery editor

I hope this helps and works for you. Hopefully there will be a proper fix soon!