This post is more of a belated confirmation than news. At least some pets who apply debuffs often do not apply them correctly to Ultraxion. I noticed this many weeks ago with my dragonhawk. Back then I attempted a fix by manually repositioning my pet closer to the boss. This did not solve the problem; my pet’s debuff uptime was still very low. It was only this past week that the stars (or, rather, raid auras) aligned again to allow me to try out another fix. This time I macro’d Dragon’s Breath to a shot, and it did the trick.
The mistake I made this time, though, was that I only macro’d the pet ability to Cobra Shot, and so my pet did not have its debuff up during the first several seconds of the fight when I’m not firing Cobra Shot. So, I would recommend macro’ing your pet’s debuff to a shot or shots you fire at the start of and throughout the fight.
I wanted to be careful about having my pet continue to behave normally in other fights (since I’m too lazy to switch macros between fights), so I used a macro line like this so that the pet stays on his assigned target and doesn’t fail to cast his debuff:
/cast [@pettarget] Dragon's Breath
If you are interested in looking up a pet’s debuff uptime on a target, you can check this under the pet’s details page on World of Logs and clicking the “Buffs cast” tab. You can then check the pound sign next to the debuff’s name to graph the uptime and see when during the fight (if ever) the debuff was not present on the target.