Some AI's include special space characters in the output. These spaces aren't normally rendered by browsers, and not visible to the human eye. But they can be detected by plaigarism checkers if you just copy and paste the text. This link has more on the issue, and how to see/remove these watermarks:
https://www.rumidocs.com/newsroom/new-chatgpt-models-seem-to-leave-watermarks-on-text
https://www.rumidocs.com/newsroom/new-chatgpt-models-seem-to-leave-watermarks-on-text