Version 1.8.5 on android
I didn't see this fixed in newer version changelogs

At the screen where you open chests with either keys or cash.
I hold my finger over the pay with cash option. for example, cheapest chest over the "7500" button
This opens the menu with the scrollbar that allows you to open more chests at the same time.
I lower the amount of chests to 1 or any number smaller than the maximum offered value that's at the right of the scrollbar.
The game gives me the reward for the maximum possible value of chests.

Try to open one chest by just buying one for "7500" -> you will get a normal reward for that chest.
Now, open the scrolling menu and lower the value to only open 1 "7500" chest -> you get far far more rewards.