From what I can tell, it isn't a bug.
They told you plainly
The amount shown is only approximate. Also, please note that all trades that are requested through “Simple View” are set as market orders.
I can only suggest you start using "Trade view" where you will be able to place LIMIT orders.
https://www.nicehash.com/my/tradeview By default, it is a market order (probably because of what 1Referee said), and that you are able to change this very easily.
Which part of this is a bug? Did you place this exchange within the Trade-View window?
From what I can tell, you just wish their default was "Limit" instead of "Market", but that's not a bug.
What makes me think it's a bug:
1. They show how much you will get. There is no word about market order, just note about liquidity, which means more time to fulfill the order! When you go to exchange money in normal exchange, you know exactly how much you will get. Here the approx is 30x, that's crazy.
2. There is NO WAY to change the order from the dedicated exchange page. Only if you go to trade view, you can decide for market or limited orders. I wanted to exchange in simple exchange, that's NOT trading view - and definitely the placing of order is HIDDEN and NOT TRANSPARENT to the user.
It's interesting how "giant" and "leader" like Nicehash failed that miserable. Not even talking about liquidity of trading of less than 1 BTC for XRP (btw all the orders were executed in less than 1 second).
Conclusion - the simple exchange functionality CAN'T be trusted at all. Moreover Nicehash CAN'T be trusted at all, same old shit with small talks, when it goes to problems.