To add to mduell's answer, the discounted/"special" ORDCDG fares have routing restrictions, they can only be used on a list of "legal" routes. You can see the list if you have an EF account, but basically, none of the routes allows MCO as an intermediary point. Therefore if you want everything in one fare, the engine is forced to select an unrestricted/"normal" ORDCDG fare for the return, which does not have routing restrictions. It's this what causes the price jump, as these fares are a lot more expensive. The YR1 fare that mduell pointed out already is an example of an unrestricted fare.

If you plan to stay longer than a day in MCO and you don't need protection for a missed connection, I'd also suggest combining the halves of ORDCDG and MCOCDG and book MCOORD separately. If your stay in MCO is short (less than a day) and you need protection, you could try adding an R/T sidetrip to MCO from a 24-hour layover in EWR or IAD (CDG-EWR/IAD-ORD is a legal route in the discounted fares). This requires a flexible ORDCDG fare that allows sidetrips, so this will increase the price again, but it should be under the $3k that you were quoted as it doesn't require an unrestricted fare. Note that you need to depart EWR/IAD for ORD less than 24 hours after you arrived there from CDG, or you'll have a stopover and everything will price out much higher again. Google Flights can help you construct this in a single reservation and ticket, but disregard the high prices that Google will quote you initially, as the total fare - after all segments are selected - may price out much lower.