Please refer the attachment.
On MacOS, Qt Sinhala Rendering is not working but Tamil,Devanagari and other indic languages working correctly. All these languages use almost same rending rules. I don't understand why Qt Text Rendering engine is not working only for Sinhala.
On Windows Operating system Qt Render Sinhala Text without any error(But automatic fallback to correct font is not working. I have to tell exactly which font to use)