"RelatedProperties" is defined to follow RFC6901-style syntax for expressing a path to a particular property. This is to allow for a service to highlight properties within other objects of a resource. However, the "#" is really just meant to be used for URIs; RFC6901 defines the syntax as starting with a "/" character. We'll need to review this further and correct examples as we find them.
As RFC6901 states, the # is used for URI fragments, and "RelatedProperties" does not contain URIs. I would expect these pointers to start with "/". I'll need to ask if others agree.
Looking at some other material, I see cases where local references are made in the URI fragment form with "#" at the beginning. This seems to be limited to usage in JSON Schema files with "$ref", which I suspect is defined as a URI. I also see other cases where tools will not allow a "#" because they're not expecting a URI reference.