[bug] HS API BG card's field imageGold is empty

Endpoint:

https://us.api.blizzard.com/hearthstone/cards/61884?locale=en_US&gameMode=battlegrounds&access_token=my_token

Response contains empty imageGold:

{
  "id": 61884,
 ...
  "battlegrounds": {
    "tier": 4,
    "hero": false,
    "upgradeId": 67728,
    "image": "https://d15f34w2p8l1cc.cloudfront.net/hearthstone/2838a6cab914136ec07b596d868bf84ef1d4be283634fce513115a3d76728c64.png",
    "imageGold": ""
  }
}

While https://playhearthstone.com/ has golden version:

"battlegrounds":
{
    "tier": 4,
    "hero": false,
    "image": "https://d15f34w2p8l1cc.cloudfront.net/hearthstone/59829e31898c5df6af84c8c8994bf5209d4e5783539c471d29fcf36c4449b778.png",
    "imageGold": "https://d15f34w2p8l1cc.cloudfront.net/hearthstone/43ff0a65d8149b37bad5199e4b2ee78f2c96d7c4740f690d0430aa561e88f68a.png"
}

playhearthstone com uses:
https://api.blizzard.com/hearthstone/cards?gameMode=battlegrounds&ids=63273%2C67728&pageSize=200&locale=en_US

P.S. (inspected with chrome dev tools)

Expected behavior: first request (from documentation) return golden images.