When exactly is PlayerPrefs.Save called?

We're building a game on Windows (Unity version 2021.3.5f1) , that saves user data using PlayerPrefs (yes, we know it's not a good idea). We found a bug that we suspect comes from PlayerPrefs not getting persisted if the game crashes and OnApplicationQuit() is not called. We only call PlayerPrefs.SetString(), but NOT PlayerPrefs.Save.

The documentation for PlayerPrefs.Save states that:

Unity saves preferences automatically during OnApplicationQuit()

https://docs.unity3d.com/ScriptReference/PlayerPrefs.Save.html

So this would be coherent with our theory. We suspect however that it's the not full story:

1. In the cases we have, some of that data gets saved, and some is lost, which means that there are "interemediate" saves of the PlayerPrefs occuring

2. Some other forum posts suggest that there is an undocumented "autosave" mechanism for PlayerPrefs: https://forum.unity.com/threads/playerprefs-autosave.1081307/

3. When I log the content of the registry key immediately after PlayerPrefs.SetString() (using RegistryKey.GetValue()), it has already been updated, even though Save was not called.

It looks like, at least in Editor and Debug builds, the PlayerPrefs are serialized to the registry immediately after PlayerPrefs.SetString(), and not only when OnApplicationQuit() is called. What am I missing? Thank you very much.