Community ManagerKatrinBlue You can find the info in a lot of places where the post isn't a year old. Here is the announcement for last fall's event; you'll see a reference to the account owning the items in the FAQ:
Reaper’s Rewards Event | EA Forums - 4967458
The first event's items were added to the base game due to massive issues with retaining the data.
You could also just test it yourself: Remove the file and see whether the items disappear. Even if they did, restoring the file would fix the issue.
It should be noted that interim progress is now saved to accountDataDB.package. This means the tasks you've done from a given set before you complete the set. I know this because I've tested it myself, as have other helpers here and elsewhere.
