That's pretty weird...the scroll should have fixed the level loss.
Maybe you should try going to a temple and have the priest cast Restoration on your thief. If that doesn't work, you probably have a bug. In that case, you'll need a more rigorous solution, like killing the character and raising them. Of course, if the character is the main one, i.e. the PC, you cannot kill them so in that case, maybe you should check and see if you have the latest patch installed...
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Melusine, Archbabe of the Order of the Holy Flame and the Laughing Hyenas, &
Official Entertainer Elf of the BG2 Bar
Your voice is ambrosia