Thank you sherman for your information and examples.
So ligature display can vary according to the font (Alegreya > Georgia), the renderer (RMSDK > creengine (for koreader)), the subsetter (to be determined) and probably some other - yet- unidentified factors.
The subset of a defective -on this regard- font, will not display ligatures. The subsetter has nothing to do about it. That's why it would be unfair to speak about a "bug" of the subsetter without providing an example. Examples will help us ascertain the origin of the defect.
It's not a test.
