There are stories about bodies of Christian monks that did not decompose for a log time after the dearth. Modern take on it attributes it to the climate in caves where the body was put after the dearth. But another important part was diet. Often the well-preserved bodies were of those who had eaten only rough bread and water for months and years before the dearth.

So I suspect both of the factors are at the play here as well.

Also related: Mystery of the Tibetan Mummy (https://www.youtube.com/watch?v=gCkB9daCVZ0)

I read an article about this years ago and came away with the impression that they were effectively mummifying themselves before death. From the Wikipedia page on sokushinbutsu:

In medieval Japan, this tradition developed a process for sokushinbutsu, which a monk completed over about 3,000 days. It involved a strict diet called mokujiki (literally, 'eating a tree'). The monk abstained from any cereals and relied on pine needles, resins, and seeds found in the mountains, which would eliminate all fat in the body. Increasing rates of fasting and meditation would lead to starvation. The monks would slowly reduce then stop liquid intake, thus dehydrating the body and shrinking all organs.

https://en.wikipedia.org/wiki/Sokushinbutsu

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...