What is a positive expense? Shouldn’t that be income?
Certain credit transactions, such as a refund on a purchase, could be assigned to an expense category the same as the original purchase transaction. For example, if you received a refund on an item you purchased (e.g. flight ticket), that transaction would still be categorised as a 'Flights & Transportation' expense. It would, however, have a positive value, ensuring the aggregated view in the Cash flow and Categories features is correct in the last 12 months period.
To exclude these transactions from Spend Insights, you can categorise them as Transfers.