I have often noticed that UVs are get cut after the mesh is exported from Unreal. So far I have accepted this, but now I have taken the time to investigate the problem a bit more.
It seems as if Unreal cuts uv shells that are beneath other uv shells (overlapping UVs) into single pieces (but keeping them in place) when importing or exporting. At first I thought it might be related to the algorithm of “Generate Light Map UVs” during import, but even if this feature was disabled during import, the UVs are cut after the mesh is exported from the engine. Funnily enough this is not the case if the same mesh was imported as a skeletal mesh.
Usually this doesn’t bother me, but it’s a serious problem when it comes to store assets or assets from Epic. In this case you can’t access any source files and have to rely on an export from the engine if you need to edit the mesh.