The phpBB 2 to 3 converter might blank a few posts with non-ASCII characters

Today I received a report from a user on a forum I host that a couple old posts appeared blank. These were made before I migrated the forum from phpBB 2 to phpBB 3, a transition that was made tricky by a problem with non-ASCII character encoding. Examining the phpbb_posts table showed that the affected posts' post_text had been replaced with <?xml version="1.0"?> plus a linefeed. Consulting a backup of the phpBB 2 database, I saw that both posts originally contained non-ASCII characters that get encoded into multiple bytes by UTF-8.