THIS IS NOT RELATED TO THE "AT PLAYBACK END..." SETTING IN THE TIMELINE PREFERENCES.
The issue I'm having is the playhead seems to jump to the timeline start when quickly pressing shift+space. I'm not totally sure if this is what's causing the playhead to jump to the start as I can't easily reproduce this error. I've checked all the keyboard shortcuts and can't find anything that would be causing this.
Seems to happen when I'm moving my fingers quickly and still have shift held down for a split second too long and then press spacebar to start playback. It's pretty annoying because it happens semi-frequently and I keep having to find my place on the timeline after it randomly jumps to the start.
AGAIN, THIS HAS NOTHING TO DO WITH THE PLAYHEAD BEING AT THE END OF THE SEQUENCE.
In the default keyboard shortcuts Shif+Space Bar is tied to "Play in to Out with Preroll/Postroll" and since you have no In or Out points set it's going to the beginning of your timeline. If you search that up in your Keyboard Shortcuts you can delete that shortcut key combo and you should be good.