Hi,
This is expected behavior: you can only seek properly using the app (Now Playing tab, seekbar).
The reason is technical: transcoded streams cannot technically be 'http seekable', that is seekable with usual http Range requests.
This is the normal way to seek in http streams and what the Chromecast do when you use the remote.
To address this limitation and still make transcoded videos seekable, the app implements an alternate way to seek which is
to play the video as a new video starting at the seek location (while making it look seamless in the Now Playing tab for the elapsed time and total duration displayed).