I think it’s maybe a little but of both of what you and Annoyed_[Crabemoji] said. From what I remember of baking, butter being not chilled enough will cause it to be too soft and cook out before the chemistry can happen and they deflate like that. But obviously, it’s real tough to mix in chilled solid butter, so by the time you’ve needed it enough for it to incorporate, it’s warm again. When I was in culinary school back in the day we’d bake in huge batches, obviously, so we’d use big ole mixers to combine the cold butter quickly with giant mechanical paddles that forced it to combine while still cold. But at home, if you have to mix by hand and you know that the butter isn’t cold anymore you can definitely chill the cough before baking. I don’t remember much from those days (I was never a baker, I was a line cook, but baking classes were required), but when I saw your picture my immediate thought from the dredges of 20 year old memories was “That butter wasn’t chilled.”