I cannot answer your question about why this happens, I can only speculate.
I think the game reads a resource square as occupied somehow, and when you try to enter from any any terrain other than a road it blocks that movement due to the "occupation."
As roads allow stacking of units, going from road to road with resource over it does not have the issue.
Only speculation though, Mark as the designer would have to comment. As you say, I don't think this is covered in the manual.
Cheers :)