I write a small IVR in javascript to react on user's 1 key input

session.streamFile("a.wav!b.wav",dtmf_handle);

when user press a key at the end of a.wav and the beginning of b.wav, there is no dtmf call back. if another key is pressed after that, the dtmf_handle is call with a event containing 2 digits.

Is this a bug?