Osmos
The 2009 puzzle game Osmos has a couple of interesting procedural generation features. First, it has an infinite-level mode where you can continue to play new versions of the levels. But the second part is what makes that possible in the first place: all of the levels were built with procedural generation.
The canonical version of each level is generated from a fixed seed, but it uses the same generators as the “forever” levels.
This is a pretty effective way to get the best of both worlds: a flexible generator that lets the designers rapidly iterate on levels while not caring about the details, while also being able to carefully manage the difficulty progression. And, of course, once you have the working generator, you can just vary the seed to get levels that feel similar but are different enough to be a new-ish puzzle.
It’s also one way around the 10,000 bowls of oatmeal problem: the game comes with 47 flavors of oatmeal, and when players request a new variation they’re explicitly asking for a variation on that particular flavor. They want a semi-predictable outcome. The player’s expectations for the generator line up with what it can produce.