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.