I also have a 2i. I have not used autofill, so take this with a grain of salt, but it appears to me that its function is not related to the DAC.
By default, BluOS has the Autofill feature enabled. With this feature enabled, when you select a song/track from a streaming service or local library, the remaining songs/tracks that follow in the album or folder are automatically added to the play queue and allow continuous music playback.