OK I now understand what’s going on, and why myself and others are getting this issue.
My problem was that I was re-saving the whole 3Ds file as an fbx, then doing the re-import, but what I needed to do was only export the single mesh that I wanted to update in UE. Now when I export say box1, and re-import box1 in UE, it’s all great.
This is more about understanding the workflow, rather than a bug. I thought I’d post this for future reference anyway in case anyone else gets stuck!