John, the precise order is as described in my original post. I shoot tethered to LR. Then convert to DNG, using "Library>Convert to DNG". Then copy the folder containing the DNG's to an external hard drive, and hand-carry it to my desktop computer, where they are imported into my LR catalog, using the "Import" button at the bottom left of the Library module.
Looks like the crucial step I've been missing is the "Save Metadata to File" - I'd never heard of that before, thanks. I also think I'm going to start using a new catalog for each tethered shoot, and use the Export as Catalog >> Import from Catalog, and see if that improves things.
The thing that makes this even weirder, in my view, is that on closer examination, it turns out that *some* adjustments were brought along in the transfer - notably some Gradient Tool adjustments. But White Balance and Toning adjustments were dropped completely.
We'll see how it goes this afternoon, thanks for the input everyone.