@Kai_Hof This is a default behavior in SharePoint site. By default SharePoint uses permissions inheritance hierarchy like:
Site > Library > Folders > Files
So, if you create any folders inside a library, it inherits permissions from which by default inherits permissions from SharePoint site which has Read permissions for Visitors group by default.
For your requirements, you can try:
- If you want to remove visitors permissions for all newly created folders automatically, you can break permissions inheritance at library level and remove access of visitors group. check: Customize permissions for a SharePoint list or library
- If you want to remove visitors permissions for specific folders after folder creation, you can break permissions inheritance at folder level and remove access of visitors group. check steps in my answer at: SharePoint site permissions overriding file permissions
Please click Mark as Best Response & Like if my post helped you to solve your issue. This will help others to find the correct solution easily. It also closes the item. If the post was useful in other ways, please consider giving it Like.