There seems to be some inhibitor on Titans reaching higher light levels. I thought at first it was just my Titan but some of my friends and other Destiny 2 players I've played with have said the same thing. They either can't go higher or it's nearly impossible. The milestones don't seem to help with it. Since we're supposed to get powerful gear. But for higher level Titan chest armor and Titan marks it is nearly impossible to get because all the ones that spawn from engrams are such a lower light level that it's useless. And no matter what light level you are you get gear that's a lot lower a light level that you are. My warlock is 335 but I'll still get stuff that's 324-23 and the occasional 327. It shouldn't be so hard for people to level up their guardians that they're having some invisible barrier that keeps them from getting actual good gear that can help them reach max light level.