This issue is fixed in the next update
Kingdom Power Level 9 bonus shows as 20% tribute bonus rather than 30% in the "Kingdom Level" menu of the Kingdom.
Note: This is visual only, and the Tribute's bonus amount will still be delivering 30% tribute bonus to your game if you have reached Kingdom Power Level 9.
Fixing the problem
As this issue requires an update to the code in the game application, the fix must go out with a new game update to the App/game stores for each platform. The issue will be fixed in the next update, 3.6.