Hi Jakob, and thanks very much for the answer.
In the five six years that have past since originally posting this I have encountered the problem again, and even created a second post where this was explained to me, so I’ll link that here.
Your solution is exactly right, or at least the best one I have found to this issue.
On a side note: I have only ever encountered this problem when importing game models (Farming Simulator, FS, Euro Truck Simulator, ETS, and the likes), but have not gotten further than that in understanding why these custom split normals are poorly imported.