Short answer
I think ethical issues should largely be covered by the standard guidelines etc for each particular product or service. However, I do think it is possible to have some sort of overarching guideline (sort of like the Hippocratic Oath) that UX designers could use as a starting point.
Long answer
The topic/issue of ethics in UX is extremely broad.
User experience, at its core, is all about the experience a person has using a product or service – be it a website, computer software, computer hardware, an office chair, a motor vehicle, a help desk, personal service, etc - especially with regard to how easy or pleasing it is/was to use.
Considering the broad nature of goods and services, it seems to me that it would be almost impossible to cover the topic of ethics in UX in a way that becomes a single authoritative source. For example, as good as it is, the Ethical Design Manifesto is all about technology. So, how would this relate to the user experience provided by a service that relies solely or predominantly on human interaction (e.g. a concierge at a hotel, a tour guide, an airline hostess, etc)?
To expand on this, the international standard on ergonomics of human system interaction, ISO 9241 defines user experience as "a person's perceptions and responses that result from the use or anticipated use of a product, system or service". According to the ISO definition, user experience includes all the users' emotions, beliefs, preferences, perceptions, physical and psychological responses, behaviours and accomplishments that occur before, during and after use. The ISO also lists three factors that influence user experience: system, user and the context of use.
Now, let’s use ISO 9241 as just one example. A good summary of ISO 9241 is available on Wikipedia. This provides a lot of detail around visual display units, keyboards, workstation layouts, postural requirements, input devices, menu dialogues, accessibility guidelines, and so on. So, if one assumes that a big part of the ethics behind UX design for these types of systems is to meet as a minimum the international standard (if it exists) for a particular product or service, then it’s easy to see why, to use your words, there appears to be no standard guidelines or documents (or manifestos) that is commonly cited or reference that is specifically for UX design.
So, to answer your question, should ethical issues in UX design be covered in a separate topic, or is it already covered by all other design ethics topic? I think it should largely be covered by the standard guidelines etc for each particular product or service. If there is no international standard (e.g. for how a call centre operator should interact with a client), I imagine that’s where best practice, determined by a combination of the market and government regulation, comes in.
However, I do think it is possible to have some sort of overarching guideline that UX designers could use as a starting point in how they approach any particular design. Whether this is something like the Hippocratic Oath historically taken by physicians, or something else that reminds UX designers to ensure they design their systems, processes, etc in a way that meets international standards or other guidelines (where applicable) and in a way that is functional, reliable, intuitive, safe, easy to use, etc, is a good debate to have.
Regardless, I think a key goal for UX designers is to always strive to continually improve upon their designs. After all, many of the objectives of a good UX design can contradict each other in certain circumstances. E.g. there would be countless processes that diminish the user experience purely because the only way to ensure a user's safety is in fact to place certain restrictions on them. But, over time, we can improve these systems so that they remain safe and improve the user experience as well.
[EDIT]
I've come across a range of external resources relating to ethical considerations and experiences, so thought I would add these here as a bit of a reference source:
