If I recall, Sustain heals the pet at a rate based on the Summoner's HP for a few tics... why it works off the Summoner's HP and not the pet's HP, I don't know but that could be part of the problem. Sustain, along with Physick, just aren't scaling well with Titan's Hp that, by design, is larger than mine and while the difference in HP may be proportional, sustain and Physick just don't keep up with the incoming damage. The difference between before and now is when I tackle open world mobs with Titan at around my level, he usually has a limited lifespan, and the only reason he lasts through most of my solo encounters is because, along with using Sustain, I manage to kill targets before they kill him... but his HP bar will be like only half full from a quick encounter, like with fighting 2 mobs in Azys Lla, or for an extended, solo target high difficulty encounter (yet one still designed for a single player) he'll be at 25-50% and that's where just one Physick, at most two, on top of Sustain would have made the difference but now it sucks and Titan wasn't designed to be a dispose-a-pet with every fight being a doom/reprieve type encounter. I've had to premptively swiftcast a new Titan and use quelling strikes when I've had its hp get so low I knew I was about 10-15 seconds from becoming the next thing on something's lunch menu.