I agree that the examples given in Rules of UEB and Guidelines for Technical Material can leave us transcribers feeling frustrated.
No one at the National Braille Association is in a position to explain why the codebooks are they way they are, because NBA is not an organization that creates or maintains any codebooks. The Braille Authority of North America and the International Council on English Braille are the decision-making bodies for braille codes that are used in the United States and Canada. NBA is a training organization.
For what it is worth, the last example on page 32 of the Guidelines for Technical Material does include complex fractions.
In addition, we volunteers at NBA are open to members' suggestions of topics to be covered in future trainings and professional articles.
Thank you for your input and for the hard work you are doing in the braille field.