Net worth over time report mystery mismatch

Quicken Mac 5.15.3 Build 515.33035.100

In a Net worth over time report, the value of one investment account for each month after November 2019 was consistently off, more than $100k over the actual value, which was displayed correctly in the account sidebar. The excess turned out to match the amount of a sell/buy transaction in the account -- one security (call it "A") was sold, another ("B") bought for the same amount on the same day, no change in account value. After I deleted and replaced the Sell and Buy transactions, most of the excess value disappeared from the Net Worth report.

Most, but not all. Now the report's value for this account was over by the amount security B had lost since I bought it, about $4k, despite several interim prices in the price history. I deleted and replaced the only other transaction involving security B since November (a tiny sale of a share fraction to cover fees) and most of the remaining excess disappeared.

Most, but not all. Still over by about $14, an amount I couldn't correlate with anything. I ended up replacing all the transactions that had occurred since November, regardless of the security involved, and now the total's over by just pennies. I'm calling that good enough and moving on, baffled as to why only this account is off at all. Even other investment accounts involving the same securities are reported correctly.

Anyone run into something similar?