Songs in playlist start in middle.

As the title says, sometimes a song will start in the middle. Probably where the last listener stopped playback.

I KNOW there is a option for this, I remember enabling it, but now that I want to disable it, I can not find it.

Edit your audio library
https://support.plex.tv/articles/200289266-editing-libraries/

go to the Advanced tab
there is a checkbox “Store track progress”
disable it