When using text-based chord charts, OnSong uses the concept of an "original" version of a song that can be modified without affecting the original content. This is to allow one song to be used in many places and formatted or adjusted without duplicating the original file.
When you open the Song Editor by tapping on the "A"/cursor icon in the Menubar, you see the original version of the chart as imported or written. When you make changes to metadata here, it will update the master version of your song. Unless you've enabled Separate Styles, this will also update the song in every book and set. For instance, if you rewrite the song into a different key within the Song Editor using the Rewrite Into Key menu, the chords and key of the song is changed in the editor, but the Song Viewer and Style Preferences Menu will also be updated to reflect this change.
Change the content of the song in the Song Editor is one-way. If you transpose the song into a different key using the Chords tab in the Style Preferences Menu, the underlying song content will not be changed. OnSong is applying transposition on top of the original version, making it a temporary change. The original version of the song remains intact.
This applies to all metadata tags that you could assign including flow, MIDI, scenes, etc. The important thing to remember is that the version of the song found in the Song Editor is the original and any changes to your set in menus outside of the Song Editor are applied without affecting the original.