I typically made a dumb reference to something I saw in the literature (JAMA, BMJ, etc) while also saying "but I guess that's just a controversial new opinion".

Basically you want to avoid directly confronting them by implying they are dumb or wrong. Instead, appeal to authority, but in a soft way, and downplay the confidence of the new result.

I had many years of being undiplomatic until I saw a video of Kevin mitnick. His normal nature was quite undiplomatic and fairly autistic, but when he did social engineering, he did an amazing job of telling people what they wanted to hear. Now, when I think something, instead of stating it in a clear and undiplomatic way, I rewrite it in a way that my mental model suggests increases the probability the listeners will change their minds and agree with me. This took a great deal of practice to be able to do in real time.

I've sometimes noticed myself giving ideas in passing during a friendly conversation where nobody is defensive, my idea is passed over initially, but some days later the person has the same idea and doesn't remember that I suggested it. This might be some kind of bias on my part though.

Point being: Offer your idea during a friendly time where it will not cause the other person to be defensive and then you have to wait. You might find that the next time you bring it up the person is more receptive.