Bitcoin Core only lists the fee for transactions in the "send" category with inputs that all belong to the wallet (ie. not coinjoined). Figuring out the fee of "receive" transactions or transactions with external inputs require looking up the spent previous outputs and summing them up.

This could alternatively be implemented for unconfirmed transactions only using getmempoolentry, which would be much more efficient. This is probably acceptable because we only really care about the fee of unconfirmed transactions.