The download performs a check to be sure the package is complete and not tampered with. I wonder if that's deleting what it thinks is a bad download and some bug or system modification is stopping you from getting the typical user alert telling you that the download is being deleted without the option to save it.
First steps, check the console app and look at
If there's no helpful item - you could always get with Apple support to troubleshoot. I often look at other logs like system.log but there can be thousands of entries so if you are looking at all the machine logs for the first time, it might be painful to search for or see the item that's relevant.
If you're game, these are the sorts of files and locations you'll move to if your App Store itself is broken and it's not a server side issue with the account or just a bad network where you can't get a reliable download of that large a size. I've had overseas networks where they mess with SSL certificates or just corrupt the data horribly that cause this.