This not only reshuffles songs I already heard, but also leaves me in a different place in the list which I assume means songs that I have not heard yet are shuffled behind the current location in the playlist.

No. Doing that shuffles the songs you've already heard among themselves, and separately shuffles the ones you haven't already heard among themselves. You don't "lose" any unheard songs or "reanimate" any heard ones by unshuffling and reshuffling.

Peter