What size is the Tiff you return to LR? I seem to remember LR has some size limits, i.e., for huge files LR will not display a preview, and/or will not even import them. I don't remember the specifics - pixel dimensions greater than 30K on one axis? or maybe just file size, a la Photoshop not saving .psd if it's say, 4GB?
Is there any chance you might have encountered something like this? (although I do remember that when LR choked when I tried to import giant .jpgs - for huge mural prints - it did show an error dialog about being unable to import them)
John