I've just spent 4 hours trying to figure this out after encountering the exact same issue.
My actual gains/losses are around $3,000 verified by both Coinbase and the CSV data.
However when imported from Coinbase (via API and CSV both) it is telling me I have a one loss of $1,352,401.39 on a single transaction and another loss of $16,842.94. In reality the loss on this transaction is less than $1000. There is absolutely no way I'm going to file with those kind of numbers.
I think something is broken?
