I didn't update my wallet to version 2.0, but my maternode has been mining. I dug nearly 1,000 coins in half a month, but when I opened the second node, I updated it to 2.0, the coin I dug later disappeared!
It is normal and you have to accept the fact.
For mandatory wallet upgrade, if you don't update your wallet, you will be on the wrong chain, which in turn help you to get massive rewards (but, unfortunately, they are unreal rewards).
Consequently, when you update your wallet later to the newest version, all your fake-rewarded coins will be gone.
If you are still curious about this, please check the masternodes.online website to see which current rewarded rate for each NYXcoin masternode and compare to what you received in the last month. I think there are a big difference for sure.