I think there is a bug in the MTP file transfer protocol. I am using Opus version: 10.5.1.0.4848
I have this little MP3 player: Sandisk Clip Zip.
Now when I copied MP3 files onto the player, the ID3 tags somehow would not work. So all the artists are listed under: Unknown.
I did some tests:
Attach player using MSC: All the ID3 tags work fine.
Attach player using MTP: Player fails to load the ID3 tags out of the file.
Attach player using MTP and transfer files with Windows Explorer: All the ID3 tags work fine.
If you set Preferences / Miscellaneous / Advanced: mtp_enable = False, Opus should fall back on using the Windows shell (Explorer, essentially, but inside the Opus window) for MTP devices, which may be a workaround.
We have only tested MTP support with Android phones, which are the main focus of it. Other devices (and some Android phones, for that matter) seem to have quirks in their MTP handling which can trip-up Opus (and sometimes Explorer, too, but obviously most device makers test things with Explorer so it's less likely).
The beta is out now (Directory Opus 10.5.1.3 (beta)) - please try it and report back if it helped. Make sure you set mtp_enable = True if you previously set it to False after Leo's suggestion.