The fact that your preceptor cannot understand why this is being said is a good thing, because it shows that you are not making mistakes because you are not asking.
Your post reminds me of a position I was in many years ago. I was an experienced peds. hem/onc nurse. My hospital closed and I was able to get another position at a different hospital. Unfortunately, my preceptor decided that I needed to learn everything over again, rather than just learn what they did different. She did not even approve of steps I took to assess my patient, and I was also a former PICU nurse. I was criticised for not asking enough questions, so was given no credit for what I already knew.
Your confidence is not something that is typically seen in new nurses, but then, many new nurses do not have the background you do. I suspect whoever is gossiping is feeling a bit threatened and territorial by your knowledge.
Continue to use your preceptor for support and hopefully over time, whoever is talking, will value you for your experience.