So this is the most relevant discussion that is still open, trying not to open a new thread.
I had the problem 2 years ago ;
Happened again this month, Quicken R52.33. I would have hoped that this problem would have been fixed by now. At least this time I was prepared and knew where to look without spending inordinate amount of time pulling my hair out. To be fair, it was an account with Elan Financial Services, which Quicken has had a problem with for several weeks now. None the less, makes me fear that it will happen again.
In my opinion Quicken should never change the opening balance without asking the user provided that the account isn't a new account. That really is the only time Quicken should make a guess at what the opening balance is, when it is a new account and not all of the transactions are available for downloading.