If you had no coin balance, it seems like that buy limit order ignored leverage limits (it opened at ~7x). Which would be a bug.
Try writing at r/bitcoinmarkets on reddit.
Probably the same thing as this
There is an interesting bug which gave me an initial margin of about 7:1, which was very interesting, it came with the delayed order filling, where order was accepted and filled but did not appear in the status for many minutes and when it did, I was short way more litecoins than I could afford even on the maintenance margin (or maybe just that amount?!)
that was interesting...