In reply to Where could one contact Cambria and Calibri authors regarding an issue?:
A subset of combining mark characters are supported in Constantia and many other fonts without mark-to-base GPOS support in order to be able to create precomposed diacritic composites using these glyphs. It would be possible to do the same thing while leaving these glyphs unencoded, but that would result in a bigger fail when these characters occur in text, since the font wouldn't be able to display them at all.