Ash and goo piles does disappear with reset of the cell. But that's does not mean they are removed physically, game engine likely simply removes any data made of player changes in the cell and when player revisit cell, game engine reads cell from original data file.
Cells which newer gets reset to original data, like settlements, newer get piles removed.
Objects in the game like this are made of instances. That is objects are constructed from a more "primitive" data structures. Say general object >> living organism >> humanoid >> bandit. Another would be: general object >> living organism >> animal >> mutated dog. And then: general object >>mechanism >> aircraft >> BOS vertibird. You see bandit, dog and vertibird share some data. This sharing makes programming easier because you don't need to code every object in game from scratch, you just add code on each level. It also allows you to interact with objects on different levels. Say you have function which acts upon all living organism. You can use such function on a dog and bandit without making two separate functions, one for dog and one for bandit. It also makes sure that particular function can not act on vertibird (say function which handles thirst).
Somewhere inside game engine there is function which handles removal of objects from a cell, without that cell being reset to original data. Objects like dead bodies for example, or junk picked up by player. My wild ques is, that this function can not handle removal of ash and goo piles because they are on a different level or branch of object structures and function simply does not recognize them. Once more, that's just my wild guess.
I ques the difference is, that bodies and junk are dynamic objects. They have physical properties attached, can be moved, interacted with, have gravity. Piles are most probably static objects. They can't be moved, they does not respond to gravity and so on. Static objects tend to be much lover in the data structure hierarchy then dynamic objects.
But those are my speculations. As I said, fact that this problem is persistent over several years signals that it's not just a bug that can be fixed, it's structural problem or oversight inside engine itself which can not be fixed without rewriting of the engine.
Why developers did not bother to fix it when creating new or modifying old engine for Fallout 4, I have no idea. If the piles are static objects as I expect, they probably does not pose big problems to performance and thus this problem simply did not seem to be worth of the trouble and time to be fixed. It's also possible, that settlements, that is cells which does not reset were not planed when engine was being developed and they were added to the game only after engine was already done.