After a few experiments with HTTPCookieStorage.sharedCookieStorage , it seems that the behavior for the storage in that scenario involves some kind of OS flushing / synchronisation:
There seem to be cases when the storage is updated, yet the updates aren't visible. Examples are: update by a process not visible by other processes from the same app group, hard crash / killing, etc.
This all led me believe that there is some caching involved (of course, i can only guess, as the documentation provides 0 clues on internals).
In some cases, such as authentication session cookies, this could have disastreous consequences (such as login out a user for no reason).
=> Is there some kind of way to force the flushing on the cookies, for the important ones ? Are there recommended alternatives for storing authentication cookies shared accross apps and extensions (keychain ?)