@spacesurfer: I wasn't able to reproduce this issue with the 64bit Sigil Windows version no matter which CodeView font I selected.
This is most likely a Windows font mapping issue. Do you have fonts with Gujarati support other than Arial Unicode MS, Nirmala UI or Nirmala UI Semilight installed and/or are you missing
Arial Unicode MS?
If so try to temporarily uninstall all fonts with Gujarati support except for Arial Unicode MS to see if this fixes the problem.