There is a program called LDB Manager (LDB means lyrics database) that stores lyrics in USLT tags. It uses somewhat non-standart format (not just text, but also timestamps, and a header), and Mp3tag can't really parse that.
But the real problem is that after I hit "Save" in Mp3tag, most of that USLT tag is destroyed - only 32 bytes saved, the lyrics gone.
Is it possible to keep that tag saved "as is", without changes?
Hex views of the file: