So, I looked on reddit, and inside it won't unless you kill the titan like I've done, but if you're directly on top it will. So maybe the damage peak is directly under the warlock, giving it enough to destroy the bubble, but does less damage the further out it gets.

If so, and it's directly destroying a bubble, than something else must be going on besides pure damage. Because it doesn't do a lot of damage. I know I've popped it inside a bubble only to be shotgunned to death a sec later, and I've done it where I killed the titan making the bubble go away. But I've never been able to just kill the bubble with landfall from outside.