Every time when I level up a champion I level up my acount level. After this I have 2 chests to open and when I click to open them, there appear only one chest. Why ? 
Try swiping to the side
dude I thought I was crazy but I have the same bug as you.
It happens when you finish a quest and lvl up your account it seems like it gives you 2 chests but you only get one
I think normally you would only gain 1 so it must be a visual bug
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