As far as I can (quickly) tell it takes some time before the copy has been written to disk, and if you try to open it before then it crashes because the patch forces it to reload from disk, which it can't.
Somehow patching LH to wait on the save or cache copied levels looks like it would be a pain.
If LH has been patched, and you duplicate/copy level, and then quickly open it, it crashes:
As far as I can (quickly) tell it takes some time before the copy has been written to disk, and if you try to open it before then it crashes because the patch forces it to reload from disk, which it can't.
Somehow patching LH to wait on the save or cache copied levels looks like it would be a pain.