But if I push and hold the voice-over button to start the recitation of the playlists and podcasts
Yes. That's what it should do.
the function of the Next/Fast-Forward button will no longer let me navigate to the next track in a playlist.
Yes, that sounds correct as well. Once you press and hold the VoiceOver button, you are in the mode where you need to select next PLAYLIST you want to use, not select the next song.
This article says, "When you're listening to the playlist menu, you can press Next/Fast-forward or Previous/Rewind to move forward or backward in the playlist menu."
The "playlist menu" is the list of playlists (and podcasts) that VoiceOver is reading off, not a list of songs on a the current playlist. So, once VoiceOver starts reading off the list of playlists (and podcasts), pressing Next and Previous buttons lets you move quickly through the list of playlists (and podcasts) to find the one you want to use. Then, pressing the Play/Pause button (when VoiceOver says the desired playlist name) lets you select that playlist. After the playlist is selected the Next and Previous buttons will again let you go to the next and previous song (on that playlist).
So, based on your description, I think it's working normally...