Dave,

That's how default restrictions are designed to work. When you default-restrict a Post Role, that means the restriction is applied for all posts unless explicitly unrestricted per-post. (i.e. the role will not be honored unless explicitly assigned per-post). If that makes sense for most of your posts, you could set Default Post Roles to undo the restriction for new posts.

Or if new posts will be in a different category, you could use Category Restrictions / Roles instead. In that case, restrict the Post Reader role for all categories the existing posts are in, but assign a Post Reader role for at least one category which new posts will be assigned to.