After further investigation, I found that Banshee doesn't suffer from this problem. It lets me add metadata in Unicode. After I save it Rhythmbox automatically loads the new metadata. This is just a workaround for now. But we would all like to have this fixed in Rhythmbox.
Rhythmbox developers might want to check the backend code of Banshee to see what is it that they are doing that is different to Rhythmbox, in terms of Unicode tag editing.
After further investigation, I found that Banshee doesn't suffer from this problem. It lets me add metadata in Unicode. After I save it Rhythmbox automatically loads the new metadata. This is just a workaround for now. But we would all like to have this fixed in Rhythmbox.
Rhythmbox developers might want to check the backend code of Banshee to see what is it that they are doing that is different to Rhythmbox, in terms of Unicode tag editing.