The Cub can cut in reverse, page 18 of the owner's manual. Just turn the key 1 click to the left and push a button, it will stay in that mode until you get off the seat, or shut the deck down.
I've had my Cub for over 6 & 1/2 years, and the only time my PTO has cut off was when I didn't look down to see if the button was activated, and lit. You can't fault Cub for something you did or didn't do. You did just admit you forgot to turn the key and push the button.