What you were expecting to happen, and what actually happened?
My total “Gold per Day” income according to the kingdom screen was above 2000. I upgraded the power level of a few kingdoms from 1 silver star to 1 gold star to get double tributes. My “Gold per Day” income dropped to 1780. Looking at the “Income” panel of the kingdoms I upgraded to 1 gold star, the gold bonus for “Kingdom Power” is displayed as +0. Those kingdoms still at 1 silver star display it as +50.
What are the steps to make it happen again?
Upgrade a kingdom power level from 1 silver star to 1 gold star, see your “Gold per Day” income dropping by 50.