Casting to my Yamaha receiver (RX-V485) works, but scrolling through the track does not. I had the feeling it worked in the pre-15 releases (but am not 100% certain since a quick downgrade showed the same problem also in the last 14 version).
Your logs show the Yamaha accepting the seek commands but continuing to report the old playback position. For example, it accepts a jump to 1:15, then reports 0:30. That explains why the slider jumps back.
Could you try casting a local copy of the same MP3 from Symfonium to the Yamaha and check whether seeking changes the audio? That will help me narrow down whether the problem depends on the Navidrome stream.
A file downloaded to your phone is exactly what I meant. The new log confirms Symfonium is serving it locally, and the position still jumps back after seeking. This also happens without Navidrome streaming the audio.
Could you try casting that same local MP3 to the Yamaha with another UPnP app, such as BubbleUPnP, and tell me whether the audio jumps when you seek? If it works I’ll the bubble upnp logs to compare.