Lashon harah can be false according to Chazal:Sde Chemed

The Sde Chemed below shows that both true and false derogatory statements are also called lashon harah by Chazal. In contrast the Rambam and others say that lashon harah is true while false statements are called motzi shem rah. This  lack of concern for precise definitions by Chazal seems to imply that  they were primarily concerned with stopping harmful speech - rather than defining its parameters as they did for violating Shabbos or stealing (I have mentioned this issue in previous posts 1 ). This focus on the moral imperative rather than the legal dimensions might also explain why the even the Rambam was not careful in formulating the definition of rechilus according to the Kesef Mishna and Chofetz Chaim. It also explains why there was no Shulchan Aruch of the issur of lashon harah/rechilus before the Choftez Chaim.

   Sde Chemed Lashon Harah Lamed 63