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.
Whether it's bug or not, why the hell you choose nicehash considering you have decent amount of btc to trade? Because they have lesser withdrawal fee than other top exchange sites? I doubt that.
This has been the issue for small time exchange sites, first you trade with a little then everything went smoothly and when you finally decided it's a good one, you'll trade bigger amount then boom, they'll hold your funds.
Anyway, thanks for the heads up.
The reason was to use their new functionality - simple exchange.