Before I bow out, can you confirm the following...I just want to make sure I understood you correctly.
1. When you create a static playlist and add songs to it, is the last added song not assigned the highest number in the playlist?
2. When you sort (descending order) the playlist with that '#' field, is MB not showing the song with the highest number on top?
btw, you never stated what version of MusicBee you are currently using (help > about).
Edit:
All along, I've been testing this out using the "select file > right-click > include in playlist" method to add songs to the playlist.
Having read your start-post again, I just noticed that you use the "drag and drop" method instead - which assigns numbers based on where you've dropped the song.
So it's very important that you drop the new song at the very top if you want it to remain there. I suspect you haven't been careful about this detail of late.