This evening I contacted support as I thought I'd encountered a bug - but it turned out to be a feature I'd never heard of before, Party Playlists, where the mix is customised so the start and end times are different!
I only found out while on the support chat via this reddit post:
The party site itself seems down: https://www.spotify.com/us/party/
If this is a defunct/deprecated feature, could playlists be 'tidied' to reflect this? I.e. reset them to play from 0? Or if it is a party playlist, then simply display a badge/UI flair of some kind to indicate there is something **bleep** about this playlist and it's not a bug!