If you lose a showdown you can choose to "muck" your cards. This means that you choose not to show the other player your cards. You can only muck if your turn comes after a winning hand in the showdown. You can also choose to "show" your cards even if you have lost.
If you muck your cards at showdown, the hole cards will still appear in the hand history. This is not a bug. Players in a live casino can ask to see what their opponent folded at showdown and this is our online equivalent. You can choose to muck the cards at the table, but any player that is interested to see what hand you held can check the cards in the history. This is ONLY for hands that are mucked at showdown.
If you have "auto muck" selected then when you lose a showdown your cards will automatically be mucked if possible. "Auto muck" is turned off by default and can be turned on/off in the "options" panel.