It maybe corking. Maybe it got planted too deep in previous container. Try to apply some cinnamon and keep the plant very dry. If you can poke or feel the crevice, see if it is too soft or hardening up. Hopefully it is hardening up so it is starting to heal. If it is continuing to go soft, and mushy, not good. But usually as long as you allow the plant to go warm and dry, it will callus and heal itself.
