I don't know but I have the same problem with quite a few messages, so I think it must be a bug. Sometimes they have a specific 'seen at' time on the computer, but then it later disappears. This might still show as a check in the Messenger App, but sometimes it's gone back to a curved arrow. Sometimes it doesn't show as read on the computer the next day, even if it still has a tick in the Messenger App. And sometimes I know for sure it's been read, but it doesn't show as read anywhere any more. So I think you can't really trust it much.