I can confirm based on a bug thread I created a few weeks ago:
- There is no guarantee to get a bullion, it is merely a high chance.
- Myself and others have reported not getting a bullion in an entire LFR clear for a week.
- Myself and others have reported getting it in normal clears after having done a full LFR clear.
- There hasn’t been a report to my knowledge where someone didn’t get it within two full clears that week across two difficulties.
- You can’t miss it if you get it to drop, it is mailed to your mailbox.
- It appears t be very rare to go a full LFR clear as most people report getting theirs within their first handful of kills.