Resizing Text boxes causes text to disappear/flow oddly
Narrowing the text box causes text to disappear and not have the correct leading. Never have had this happen. It almost seems like it's automatically adding it's own "Space After".
Try turn on your invisible characters (Type >> Show Hidden Characters) and check if you have any "nonbreaking space" characters (shown as blue ^) between words, as opposed to regular spaces. These will force words to stay together on the same line, causing them to not fit in the narrower box.
I don't see anything wrong with the leading.