Not a bug but a change in how tunes works.
MusicSlots don't technically exist in 2.1.15. Since the game and most mods still use the O_MAPXXN format, however, tunes (for now) lets you put in the old music slot number for compatibility. However, the slots beyond 1035 were just arbitrarily mapped to names with no real rhyme or reason. Just use their actual name to access them.