Entering full screen mode always results in drop of window–with a big space between top of window and the tabs.
Welcome to the community. I don’t know anything about macOS. Just wondering if the following information relates in any way to your issue.
If you are requesting help, you should probably edit your topic and change the category to Browser Support/Desktop Support. You are more likely to get help from a community member or Brave support in that category. The Feedback category is usually informational only posts and you may not get any replies. So, if you need help, it would be best to change your category to one of the support categories. Take care.