I think it's inaccurate to say that RPOs "work for" the person they're assigned to. To the best of my knowledge, the protectee doesn't get to choose the officer, though they can state that they prefer a man or a woman. The RPO is paid by and responsible to the Met police. The Met makes the rules about how many officers a royal has and the scope of their protection (i.e., some royals only being covered when on official engagements).
Moreover, if someone "works for you", part of that is directing their work. In the case of royals and RPOs, the RPOs make decisions about the royal's security and expect the royal to comply with security decisions.
This isn't an employment relationship, and to paint it in the same light as the PA or Samantha Cohen leaving is disingenuous on the part of the papers.
And once you consider how ridiculous the story "woman who said she was going to quit but agreed to stay for six months is now at the end of those six months" is as evidence of *anything*, then you're left with "a single PA left after a short period of employment and appears to have been disgruntled."