Well getting 16k coins (visual bug), which is around 36 coins is ok.

But getting 20-80 coins, which actually seem like the correct amount to get for it to be in reality 0,17 SAGA is kinda sad. Why would the payment get decreased so suddenly when node has constant uptime?
The dev has already explained this. Please read his response again showing the blocks of rewards received.
The dev's premine is not the only one that's staking. There are other people who stake as well, so you will get a series of high (from premine) and low (from other people) rewards. At the end of the day, the average would be roughly as what the estimated ROI stated.