I'm not clear on why some are affected and others are not. Hoever, I think it may have something to do with coding. Please try this first and see it it works for you.
Go to Settings > Writing and select “ ___ WordPress should correct invalidly nested XHTML automatically” and then scroll down and click “Save Changes.”
Starting with your latest post, open it in the editor, make one minor change such as adding a space and then deleting it, and then click “Update Post.” Check your blog and see if it is back to normal.