When casting through Spotify Connect or the standard cast API, the timebar (where you can select a position in a song) and the timestamp remain at 0:00. Shown in screenshot, which was taken a minute into the song.
The song length and playback are unaffected. Album art sometimes falls out of sync when a song ends (next song plays, but art doesn't change). This affects both songs and podcasts.
Cast a song, e.g. to a smart speaker, and open playback screen.