I've been dealing a lot with page types and page defaults these days. Actually I remember myself setting up default values on some attributes (more for testing purposes than as a real need. Anyway, now that you mentioned it - the same is over here - composer form doesn't pre-populate those values in corresponding composer form fields.

If I remember correctly, it does show them when you visit page defaults for editing.

If you ask me - page defaults is a wonderful feature but very buggy.