This might have been covered already but this is the first time I have encountered this bug.

Attempting to redeem companion certificate where travel is valid only in certain classes (LUT, but also apparently also X and V since the fare class realignment).

If I make a dummy booking without the certificate, the flight I want has >2 seats available in X and only 1 seat available in V (i.e., dummy booking for 1 showed V fare, dummy booking for 2 showed X fare). Plenty of availability in LUT. When I plug in the certificate this flight does not appear at all. If I try using the certificate for another date that has >2 seats available in X and 0 available in V, the flight appears (in X).

The web help desk first tried to tell me that the problem is I need LUT availability. After I pointed out that the flight does have T availability the agent was able to manually setup the itinerary in X class and transfer me to reservations... where they tried to charge me a phone booking fee . Finally able to get that waived and it took me an hour in the end. It hardly seems worth it given the measly net amount I saved using the certificate (vs buying 1 V fare and 1 X fare).