When I use importmulti with an xprv it never imports the private keys. What am I doing wrong? When I call: 

bitcoin-cli getdescriptorinfo "pkh(tprv8iDvhA6eaMDbRuH3D2DTNuH6N6LvMTVrcv7z6sVxyyYvThSaCSa3i3qFVqUgVL4NbVkACxjx3KPWLoP5jvDU3i16hjTnzEFnZuqsnTJ5F4n/*)"


it returns:

{
  "descriptor": "pkh(tpubDEuxqa8tiiuGKNJq6ft3nJwCw7rrWngmCDimPPYGQFMKJBhLpqPdtYT7fzuDbk1AppxRdYjzeUrLaVNQRP7EYghzLDBMguY2LHHHeHMtzhm/*)#0wkkha79",
  "isrange": true,
  "issolvable": true,
  "hasprivatekeys": true
}


I then call:

bitcoin-cli importmulti '[{ "desc": "pkh(tpubDEuxqa8tiiuGKNJq6ft3nJwCw7rrWngmCDimPPYGQFMKJBhLpqPdtYT7fzuDbk1AppxRdYjzeUrLaVNQRP7EYghzLDBMguY2LHHHeHMtzhm/*)#0wkkha79", "timestamp": "now", "range": 100, "watchonly": false, "label": "Hot Storage", "keypool": false, "rescan": false }]'



It returns:

{
    "success": true,
    "warnings": [
      "Some private keys are missing, outputs will be considered watchonly. If this is intentional, specify the watchonly flag."
    ]
  }



I am importing an xprv because I want to import the private keys, but they always get imported as watch-only. 

Why is this? Is it intentional?
You aren't doing anything wrong. This behavior is expected for Bitcoin Core 0.18. The Pull Request allowing private keys to be derived from descriptors and imported with importmulti was merged recently. This functionality will be included in Bitcoin Core 0.19.