On the one hand this has nothing to do with the new (built-in) player

You’re right, I misinterpreted the mention of Ogg streams in the opening post. Sorry about that.

I always used VLC as external player together with the Ogg stream and since they are delivered by OxyRadio things just work like a charm. There is no problem to skip to any position in a song.

In VLC they indeed seem to be seekable. I’ve been using Totem until now, and with it the latest changes really did make the streams unseekable there. So something has changed, but it affects seekability depending on the app’s support for it. Apparently Totem’s streaming support is more limited than VLC’s.