Chromie is a male dragon. Dragons in WoW can take other forms. Chromie usually manifests as a female gnome. The designers said years ago that Chromie was supposed to be a female dragon but somewhere along the line someone forgot that and wrote about the dragon being male.

This is not a trans situation where someone is trying to be something that they're not. Nor is it a retcon. It's just a minor character that they didn't put enough effort into developing where people are just now noticing the lack of consistency.