Sorry for the delay but only looked into it now.
If you play a track that is not yet cached (to storage), it will not be seekable (for technical reasons).
When you play such non cached track for the first time, the Now Playing tab displays 'Caching', then 'Cached' when the track is cached,
but the original (non-cached, non-seekable) stream is still played.
Ability to seek is only possible when playing the cached version, which requires to stop playback and play it again.
Long story short: seek is not available only the first time a track is played. If it is fully cached (may not be if you abort playback before), it is seekable.