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Sounds to me like there is a combination of factors.
Guinness is fermented and has carbonation, which acts as a kind of leavening in the baked good. The actual leavening agent- baking powder (are you using baking pyrophosphate?) will actually degrade the longer it sits in the fridge. I've found that eggier batters and oil based batters will perform well when refrigerated.
I'd assume that his cake contains oil, not butter and has two or three eggs?