come to think of it, I created the cube inside another complex mesh so…
If anyone else stumbles on this, you may need to disable triangulation on export. Triangulate your mesh beforehand, disable triangulate on export, and UBX_ boxes work.
Using blender instead of wasting time just add a triangulation modifier to the main mesh and select the option to export only after applying modifiers.
This has worked flawlessly consistently - but because epic is… Well… “epic”… you do need to switch to the old legacy fbx importer as they couldn’t help them selves from breaking something (else) that has always worked…
Ergo, make sure you use the right importer for 5.x versions of the engine 