Extend function producing a new song instead of extending provided audio

Am I doing something wrong? I have uploaded a 31 second clip of a part of a song I made years ago, and when I try to extend it (from 31 seconds, from 29 seconds, or wherever) the resulting audio never begins with my provided audio and instead is always entirely new right from the beginning. This is the case whether I'm generating with lyrics or as instrumental - and when I do attempt as instrumental it always generates with lyrics & singer regardless of having toggled 'instrumental'.