after JEmplode changed the "tunes" to the symbol.

Jemplode didn't do that. It's the way the player software presents its menus. All Jemplode did was create the playlist tree the way you told it to.

But I had mixes of sub-playlists and tunes (withing one playlist) before and I can't remember ever seeing this symbol.

It's possible that this feature was only implemented in software version 2.0 and later. Although I'm not sure.