The software has been designed to support the Sinhala font with all its characters and symbols. If you have issues using other font, then it could be possible that the font is unsupported.
I recommend creating a username and password on Sourceforce to open a discussion thread on the Discussion tab. There you can report this issue to the developer and he should take care of the rest if it's possible.
Discussion forum: https://sourceforge.net/p/keyhelp/discussion/