When importing from phpBB, I get messages like the following:
Parent post pm:1234 doesn’t exist. Skipping 1235
Since phpBB doesn’t treat conversations via private messages as a single unit, it’s quite common for people to have the most recent reply to a private message in their inbox but remove the original message (the one that was replied to). While having these “incomplete” conversations dropped isn’t a huge issue, it would be more consistent to import the messages as they are and consider the oldest message from a conversation as its root.