More info:

It's a known glitch in the website, its due to the fact that UA uses the same booking class (O) for business class on a 2 cabin aircraft as they do for first class on a 3 cabin aircraft.

However AP only charges the J class points if there's only 2 cabins on the flight, so when that happens all that needs to be done is to book at 60K (it usually will ask to call to get the taxes calculates) and when calling to complete the booking, the reservation system should realise its a J class redemption and only charge 40K miles. In the unlikely event that the booking can be completed online, a simple call afterwards will allow AP to re-adjust the mileage level and re-deposit 20K miles. I've seen this happen many times, its quite common.