So I have been experimenting with just playing the playlist sorted to get a least some alphabetical randomness. I have played nothing else but this playlist just to be consistent.
As you can see below (see comments at the foot of each picture) this "Reset" issue is bigger than just shuffle play. Either Spotify is deliberately resetting the Queue (to what purpose?) or it is just some really bad coding?
If you have tried creating a Radio Station from your playlist you will also recognize the problem you see here, where the station just seems to recycle the same collection of song after a while.
Why encourage your subscribers to curate and share playlists if Spotify is thwarting your ability to enjoy any playlist over 150 songs...