Actually, I cannot really reproduce that.
What I have observed, though, is that on one machine every edited line gets blanked (except for the filename) when I have the WMP open at the same time and the data is on an external hdd, connected via USB3 and the WMP is currently updating the library - which it alwyas does as soon as you start editing.
As the WE also likes to index files, the same blocking mechanism may step into action.
So check if you see the same behaviour, when you close the WE after dragging the files into MP3tag.