Not that issue, but I've had several problems creating genius playlists. Basically I have a song playing and I tap the "genius" button, it creates the playlist with no issues, but when the song finishes it goes onto the next song on the original album instead of the next song on the playlist.
I have had the music app crash on me a few times. I'll be scrolling through songs and it will hang and just exit and go back to the main screen. I'll start it up and it will be like 4-5 songs from where I was
I assume this is an audio file that hasn't been marked to save your place (like in an audiobook). If by "idle for a while" you mean long enough that the memory used for the music app has been overwritten by another app, then this behavior seems reasonable.
For instance, if you start playing a song, then delete the music app from the "Recent Apps" list, you'll see this behavior; the app remembers the song you were on, but not your place.