Just tried it again. I have 70GB available. Got on the hotspot, started the download, unplugged the laptop, waited for it to go to sleep, woke it up - same result. I had to accept the EULAs again and it started to download from about 50mb.

Having attempted (unsuccessfully) to write a resumable HTTP/HTTPS downloader, which is what I suspect nsurlsessiond is using behind the scenes - it's really hard to get it right. Meanwhile I expect to be able to start a BitTorrent download, throw the laptop down the stairs, take out the hard drive and be able to successfully resume it on a different computer because that's a protocol that was actually designed for it.