Conflagrant Raxeal does not drop a gold treasure chest?

Yes, it is.
Gold chests are rare drops. I mean, even rarer than silver chests. I had a few UMs dropping silver, too.
Riki has a skill that raises the odds of the enemy dropping gold chests, he doesn'T need to be in the active party for the skill to work. I believe linking this skill raises the odds. But it's still not save to say that UMs will always drop gold.

Yes, it might happen. Conflagrant Raxeal isn't a proper UM, it might not drop gold chests at your first try. All proper UM's ALWAYS drop gold chests. Those that don't are not real UM's. If you notice, you don't get Affinity Coins when you fight them the first time.