You could try updating (tuenc is 2017/01/24 v2.0c in the current release) but I don't think it's likely to be a macro issue --- ligatures are handled by tfm metrics or the tex-text mapping depending on the font type. Others on this list would be better able to suggest ways of debugging a failure for the tex-text mapping to kick in.
David -------------------------------------------------- Subscriptions, Archive, and List information, etc.: http://tug.org/mailman/listinfo/xetex
