I have the same problem as you, I done some testing and figure out root cause:
- Create new class GameScore as document
- Do the same create and update object and find out it works well
- Check the "ACL" columns and compare to your problem data. See?

Works ACL, I done modify for * to make it clear

Failed ACL.
As you could see the write privilege lock to user "6iIv5XWZvM", that's why your update will "not found object"
You could change data record directly to "*" to let your update works well.
OK! Here is solution as follow:
Change every record "ACL" data col as follow:
{"*":{"write":true,"read":true}}
You need login PFUSer as specific user to change ACL to public. For my case use objectId will be "6iIv5XWZvM". I need set password and login info and use code to login and modify it. Detail iOS code as follow:
//iOS Sample
[PFUser logInWithUsernameInBackground:@"USERNAME" password:@"PASSWORD" block:^(PFUser *user, NSError *error) {
if (user) {
PFQuery *query = [PFQuery queryWithClassName:CLASS_NAME];
[query findObjectsInBackgroundWithBlock:^(NSArray *objs, NSError *error) {
for (PFObject *obj in objs) {
PFACL *defaultACL = [PFACL ACL];
[defaultACL setPublicReadAccess:YES];
[defaultACL setPublicWriteAccess:YES];
[obj setACL:defaultACL];
[obj saveInBackground];
}
}];
} else {
// The login failed. Check error to see why.
}
}];
However remember to add following setting before you save new data, if you don't want to happen anymore (note: it will be security concern)
//It is iOS code sample.
PFACL *defaultACL = [PFACL ACL];
[defaultACL setPublicReadAccess:YES];
[defaultACL setPublicWriteAccess:YES];
[PFACL setDefaultACL:defaultACL withAccessForCurrentUser:YES];