[lejuan]

After many trials to force the failure, I can confirm the problem in my case is only related to RTF files.

You get wrong characters both if the non-English ones are present at the file Properties or if you manually write them as Calibre metadata.

It is the same if you edit and rewrite the Title and/or Author directly on the device metadata fields before unplugging it (in fact, the reader ignores any manual change at this time).

When you connect the reader again, the wrong characters are present. But now, you can edit them and changes remain after unplugging the device.