Thanks guys you’ve come to my rescue again!
The book was done in Sabon, which is not redistributable, so changed it to the more generic Palatino, to make it look nicer on some readers, well I like it. I don’t usually embed fonts was just trying to push the aesthetic envelope for myself, bit like King Canute really.
That is actually a restriction of the QR rendering kit that is used by Sigil and Calibre. There is a font bug in there. It has been the case for quite some time.
Sneaky little so-and-so’s these bugs. They’ve been keeping themselves well hidden up ‘til now