0.16.23 was released on 21 September. The bug was in POP3 TOP, which Apple Mail uses to fetch headers and previews: it counted the requested lines from the first byte of the message, headers included, and reported the full size even for a partial answer. Messages Apple Mail already downloaded in that state may stay empty, so test with new HTML mail after upgrading. If the old ones are still blank, removing and re-adding the account (with mail left on the server) makes it fetch them again.