I've noticed the same thing lately with a couple of big projects I'd recently backed up. Probably 20GB~ of data across multiple files, all contained within one subfolder. I fully uploaded the files to BT Cloud using PC#1, I verified the files were all there in the web browser interace to be absolutely sure.
I beleive the problem lies with selective folder Sync.
One of the reasons I want those files backed up in the cloud, is to free up local storage space, so, on a separate device (lets call it PC#2), when I turned it on, the BT Cloud desktop app started syncing automatically and began downloading those aforementioned big files. I didn't want this, so, on PC#2 (and while the app was partway through downloading these files) I deselected those folders via the the Sync>Make only the following folders available on this computer option in the BT Cloud Desktop app.
Sure enough, I checked a few days later, and the folders I'd chosen not to sync locally on PC #2 had mysteriously had their contents deleted on the BT Cloud web browser interface (and they were now gone on PC#1 too). The folder & subfolder structure was still there, but all the files were just gone. And this isn't the first time this has happened.
As it stands, you really cannot trust BT Cloud with anything important, those backups could have been the only copies I had (and they very nearly were).
While I haven't extensively tested it, it's my belief that deselecting specific directories/subdirectories in the Sync menu is bugged and is actually at some point after, deleting those files in error after the user chooses not to sync a folder on a secondary PC.
