I had a similar problem booking USA-CDG-BLR (the only routing you can do on DL) where when I just searched CDG-BLR one-way flights I would see them available in business for 42.5k miles, but when I tried to buy a routing from USA-CDG-BLR it would offer me the TATL legs in business but the to/from BLR ones in coach(even though I was paying for business the whole way).

Eventually had to call and talk to about 6 different phone agents over the course of 3 days in order to get the ticket booked.