Why did my all butter puff pastry go soft less than an hour after baking?

I agree with Boulangere. Also - puff pastry is best when baked straight from freezer into a super hot oven - at least 375F. Color is flavor and crunch when it comes to puff. It's important to wait to open the oven door until the puffing magic has taken place, and the pastry is "set" with color. I like to cool my puff pastry projects on a cooling rack so they don't steam on the bottom. But if it's humid in your kitchen, or has been raining for days, even properly baked puff pastry will wilt. All pastries attract moisture, but few suffer as much as laminated doughs. Best of luck next time!

My thought is that it was under baked. Puff pastry is entirely steam-leavened, so it needs to be baked to a good golden brown in order to both fully raise it, and drive off enough moisture that it does not go all soft. It's also important to leave it uncovered until it is completely cool, otherwise condensation will accumulate, which will also cause it to soften.