It depends on the order of doing things, though I think to explain what you are seeing it might be safe to assume that you imported the affected images directly into LrC before you uploaded them to Lightroom while on the road....or at least the direct import into LrC was done before the images synced down from the cloud into LrC.
Until recently, if you had images imported into LrC (but not synced) and then you added the same images to the cloud via Lightroom, the subsequent sync download would detect that the images already existed in LrC and instead of copying them into the designated downloads location would instead create Virtual Copies of those images which would of course be shown alongside the originals in the original folder. The VCs would be flagged as synced and would appear in All Synced Photographs, the originals would not be synced. But the synced images would not appear in the designated downloads folder.
I say until recently, that was the actual behaviour. However I saw something odd with that process a couple of weeks ago which made me think something had changed (but not announced as far as I can recall), so I've just done a more formal test. What's changed in that "import to LrC first, then import the same images into Lightroom then sync" scenario is that when LrC detects that the files due to be downloaded from the cloud already exist in LrC, those original files are now simply marked as synced.....so they'll appear in All Synced Photographs but not in the designated downloads folder (because in this scenario they're not actually downloaded). Which is exactly what you have reported.
If you do things in a different order, i.e. add to Lightroom first, let them sync down into LrC, then import them into a different folder in LrC, you'll get a different outcome, i.e. the sync downloads will go into the designated downloads folder, and they'll be the images that are added to All Synced Photographs. Oddly, and this feels like a bug, that subsequent import into a different folder in LrC is allowed (I would have expected duplicates detection to disallow that), but after import the second set are not flagged as synced.
Hopefully that explains your mystery, and the moral of that story is that there's really no need to import the same images into Lightroom and LrC unless there's a very good reason.