Recently (last few months?), Quicken Classic Deluxe for Mac 8.2.1 has started deleting very old (5-10 years) random transactions from my register. It's only at most a few dozen per year, but it throws off my account balance and it's VERY tedious to track down and remedy (luckily I keep my statements for like forever). I get no warning of this - I have to notice that my balance is off in order to know it's happened, and then tediously check the running balance month-by-month against my paper statement to find the deleted transactions. (I know they're getting deleted because I've always kept my balances in sync with what my statements say, for many years).
Alas, I don't see any pattern, per se, except maybe it's only transactions from one particular bank (USAA) (at least that's the only institution I've seen it happen with). It might delete one transaction from, say Nov 2016, and then 6 from July 2018, etc. etc. It might be a credit or debit, it might be from any vendor. Help!