A phantom/invisible transaction is throwing off my cleared balance. What can I do?

I am using Quicken Classic for Windows, version R64.29. I have been a Quicken user for 35 years.

A few days ago, I downloaded transactions from my brokerage into an investment account, as I regularly do. When I went to reconcile the account, the "prior cash balance" shown in the reconcile window was significantly higher than the actual balance that I had last reconciled/cleared at the end of the prior month. For purposes of illustration, imagine the reported "prior cash balance" showing $10,000, but the actual balance when I had last reconciled/cleared was $8,500. So Quicken was showing a balance that was $1500 higher than reality.

I discovered that Quicken had downloaded a purported transaction from my brokerage and, without any action by me, marked that transaction as "R" (reconciled). [I say "purported" because the transaction never actually happened in the real world. I did not make the trade that the transaction purported to reflect. And I double-checked my brokerage account on the brokerage's website to confirm that it does not contain an entry for such a transaction.] Interestingly, the amount of this phantom transaction equaled EXACTLY 50% of the discrepancy in the prior cash balance. Using my example, the transaction showed a $750 inflow into my account (50% of the $1500 discrepancy). When you consider that the real discrepancy in my account was not actually a nice round number like $1500, but rather something more elaborate, I found this striking.

First, I uncleared the transaction (removed the "R"). Then, when I tried to reconcile again, I noted that the prior cash balance was now showing $9,250, still $750 higher than it should have been. Then, I just deleted the phantom transaction in its entirety. Unsurprisingly, that did not have an impact. The prior cash balance was still off by $750.

I scrubbed the register looking for a duplicate transaction but I couldn't find one. I used the Quicken report feature to look for transactions that could be causing the issue, but have not found any. I went back and checked the opening balance of the account (which I understand has triggered reconciliation issues for some Quicken users) but the opening balance checks out fine. I am confident there is no other transaction or series of transactions visible anywhere in the account (or, for that matter, in any other account in my Quicken file) that explains the discrepancy.

I believe somehow Quicken is recognizing an invisible transaction that I can't see in the register, but that Quicken is counting as "reconciled" for purposes of calculating the prior cash balance. I believe it is a duplicate of the phantom transaction that Quicken downloaded (the one that I uncleared and deleted). The fact that the amount of the remaining discrepancy is exactly equal to the amount of the phantom transaction is too precise to be a coincidence.

Has anyone encountered this issue and are you aware of anything I can do to fix it? For a variety of reasons, it would be massively painful to have to go back to a prior backup and start all over again.

Thanks very much.