I have recollection that some of the early iterations of the S905 have silicon bugs which can result in artefacts; back in 2016 the folks behind WeTek made a big deal out of their boxes only using revised silicon with fixes, but I forget the specifics now.
If you can share sample clips of media with artefacts I'd be interested in testing/comparing with a C2 and WP2; although my C2 is from one of the last production batches so might also have later S905 silicon. I don't see any issues with 1080p H264 on a WP2 box the kids use, and seek works well. HEVC shouldn't have playback issues but does have some known seek issues (as stated in release notes). SMB vs. NFS shouldn't make any difference (unless using over WiFi, which is always rubbish for streaming).