I reported this problem today via Help > Report a Problem AND a call into Quicken Support. It is the second time in 2 weeks it affected me.
Windows 11, Quicken Classic Premier R64.30, Fidelity EWC+
Following my daily download, Quicken reports a "Securities Comparison Mismatch" on a handful of securities. I found that the mismatch occurred because transactions previously reported and entered into Quicken had gone missing. Example: on 10/06/25 39 transactions were downloaded to one of my Fidelity accounts. These included Dividends, Buys, and Solds. Today (10/17/25) I did my update for transactions recorded on 10/16/25 and Quicken reported a Mismatch. Turns out, 7 of the 39 transactions from 10/6/25 were no longer in the register. I confirmed this by loading a recent backup and I could see the original transactions. The first time this occurred; previously reconciled transactions were lost.
For Quicken support; I just determined that the 7 securities that were lost from 10/6/25 had new transactions on 10/16/25. The 10/16/25 transactions were completely different buys/sells from those lost on 10/6/25.
The workaround is to find the lost Quicken transactions in Fidelity's Activity & Orders tab and manually reenter them.