OK, I solved it myself.

It seems that DVDA ignores the order in the playlist, at least as far as the Next command is concerned.

The titles are numbered based upon their position in the project overview window. In order for the Next command to work to skip to the next title, that title must appear beneath the previous title in the list. In other words, the Next command won't go from title 2 to title 1, regardless of their relative positions in the Playlist.