I have confirmed that gapless playback is responsible for this bug. Disabling gapless playback in the preferences will make the slider work correctly again. This will cause a brief (<1 sec) pause between tracks, so depending on your albums, this may be no big deal (modern music) or it may be very jarring (e.g., orchestral albums, Pink Floyd, etc.)
I have confirmed that gapless playback is responsible for this bug. Disabling gapless playback in the preferences will make the slider work correctly again. This will cause a brief (<1 sec) pause between tracks, so depending on your albums, this may be no big deal (modern music) or it may be very jarring (e.g., orchestral albums, Pink Floyd, etc.)