Thank you for sharing the issue and I am sorry about the experience!
From what I know, ARCHICAD uses Arial Unicode MS, which is a default font in Windows to write the Expression. Unfortunately, it is not possible to edit it, as it is hardcoded into the program.
If the font is missing or corrupted, ARCHICAD will try to find a substitute to replace the font, and it often goes wrong as can be seen in this case. If you happen to know which font ARCHICAD is using, you need to remove this font (and also fix the Arial Unicode MS before hand), in order to make the expression to use the correct font.