I gave it a go and imported a transaction CSV from my bank - National Australia Bank. 150 row CSV. The Description import column was assigned to the Merchant Name CSV column (the Merchant import column was also assigned to this CSV column) and as a result 25 rows were going to be ignored. Here's the CSV header row:

    Date,Amount,Account Number,,Transaction Type,Transaction Details,Balance,Category,Merchant Name,Processed On

Hand-picking the Description column to be assigned to the details column sorted it out; not sure how you're auto-selecting column mappings but maybe 'details' should be recognised as closer to 'description' than 'merchant name' is.