I don't think I'm doing anything unusual, I just press the download button and the download notification immediately shows 100% and will just keep going until I force close FKUpdater. Maybe it's something on my end? I don't have a crazy setup. Running Pure Nexus, no xposed.
Can you grab me some logs?
adb logcat | grep com.franco.kernel
EDIT: are you sure it consumed your data? Because I just tested with a malformed URL, producing the same "100%" notification like yours and it doesn't download anything even if I leave it there. I'm seriously boggled and without some logs (not sure if the logs I asked will show anything though) I can't even understand, because the URL I pass down is the same for every other user (and for myself too) and it works fine - and it's the same method I use for years, without hiccups too.