[O]ughtn't authenticity outweigh relevance?
The goal of a CVR transcript is not to give an authentic account of what happened in a cockpit. It is to give a relevant account of what happened in the cockpit.
A typical investigation board (NTSB or equivalent) has a very specific goal: investigation what happened to prevent it from happening in the future. To demand exceedingly intrusive measures into the privacy of a pilot - like installing a number of microphones recording everything they say - safeguards are put in place (remember, in virtually all other sectors, an employer installing microphones in the workplace would rightfully face serious criminal charges).
For example, the investigation typically explicitly excludes civil and criminal accountability questions. As additional guarantees to the privacy of the pilot, only a selected number of people get access to the original recordings, and they decide on the content of the transcript.
As their goal is to give a relevant account, not an authentic account, they can decide to withhold information that could be harmful to relatives (like the exact language used when their loved ones faced imminent death), if the information does not help in establishing the goal of preventing future accidents. Unless the expletive is pertinent to the investigation (for example, if it could be misheard as a specific command), it is left out.