I think you’re referring to the phenomenon named “incorruptibility”:
https://en.wikipedia.org/wiki/Incorruptibility
Recent case, died 2019, found not decomposed in 2023, had not been embalmed, unsealed wood coffin:
https://www.ncregister.com/cna/incorrupt-body-of-sister-wilh...
In "The Brothers Karamazov" by Fyodor Dostoevsky describes sort-of the opposite process. A body of monk who had been considered a living saint started to smell on the second day after his dearth. That made everybody to realize that he was the opposite of the saint. And then another monk mentioned that eating sweets and cakes could not lead to sainthood.