Are you sure you did run Full backup and not Copy backup?
Are you sure logs were not truncated? Have a look at Exchange server, Event viewer, Application log, soure = ESE, event ID = 224 at the time your backup finished. In this event you will find information about purged transaction logs:
Information Store (5876) Main Mailboxes 001: Deleting log files D:\Mailbox Databases\E000087CF90.log to D:\Mailbox Databases\E0000884665.log.
Kind regards, Jan