Playlists, as best I can tell, are sorted by track number by default, regardless of the sort of the Music Library when they are created. This is not expected and can cause unwanted effects, specifically regarding reordering tracks. Two proposals:

1. To sort the playlist based on the Library's sort (so the sort is expected and there is a precedent)
2. To cause a reordering of a track in the playlist to "unsort" the playlist.

In this case, the rest of the playlist should remain as if it were sorted (so it is a minimal change), but so that any additional tracks added while building the playlist go expectedly either among the existing sort or to the bottom (depending on the state of the playlist).