I have a playlist that I created and it plays for about 8 hours. I have it on shuffle but it doesn't properly shuffle the songs. I listen to the same songs over and over, and I noticed it did not play my newer added music. Please fix this! The playlist is 8 hours and I still end up listening to the same ones. I also noticed that it does it more on my web browser than on my mobile app (could just be me but an observation).
I believe you're just getting unlucky; shuffling is a randomized process that is designed to randomly select the songs from your playlist/library and play them back in whatever order they come. Sometimes you get the same orders; I find manually skipping the songs or switching to another playlist for a moment then return to the one you wish to listen to helps the randomization.