What could cause a sudden loss of sour flavor? Background

During the weekend, my mother made a plum cake and put slices of plum on top. She gave me a piece to try and it was sweet and good for ~1.5 seconds before my face puckered from how sour it was. I can usually take sour thing (I like to eat lemons/limes with salt and keep dipping into a bottle of citric acid), but I was surprised by how a cake could be so sour, even with a small slice of plum.

She took the slice of plum off of another piece and gave it to one of her neighbors who also puckered from how sour it was.

“Problem” / Mystery

The next morning, she had another piece and was surprised to find that it was not sour whatsoever. She gave me a piece later in the day to confirm that it was not just her tongue that was broken and sure enough, it was not sour in the slightest. All traces of sourness had completely gone, literally, overnight. In fact, she made two of them and they both gave the same results (really sour, then suddenly not).

Question

I know that in the culinary field, some kinds of food molecules can break down under certain circumstances, but I have never heard of anything that could explain this.

Now we are both baffled by this mystery. What the heck could cause a homemade cake that was shockingly sour to lose every last bit of sourness in less than a day? Nothing had changed (she did not cook or freeze it); she left it on the counter, covered in a mesh overnight.
It may have been the act of leaving the plums on top of the cake, covered overnight at room-temperature. This might have concentrated the natural ethylene produced by the plum enough to cause it to ripen by breaking down the starches and turning it into sugars.