The hard-line answer is as much about the damage enemies are dealing as it is the exact mechanic of the capstone. When you have enemies that are one-shotting you regardless of how high your shields are, I already consider that a problem that needs to be addressed.
I’ve run Moze with 2/3 and 3/3 Thin Red Line, with and without a Deathless relic on various shields, and with and without Tenacious Defense. And you know what happens? Regardless of the build, there are enemies that randomly deal hits that cut through all of her shields and even if she has over 50% health, puts her right into FFYL.
Not fighting Wotan, or Scourge, or any other boss, but just random mobs who are capable of dealing a sudden explosive spike of damage. And since enemies aren’t supposed to be scaling anything but health, armor and shields with Mayhem levels, that means it’s a factor of their own base coding for dealing damage to the player. That in and of itself sounds like a bug.
With Moze the issue is incidentally compounded if you go red tree. The devs have preached high and low that they want build accessibility and variability. But if you have to only structure the use of the tree ONE given way for it to work, and it still doesn’t perform the way the skills say it should, that is a failure on their part that needs to be fixed.
Shield-gate is literally the concept Tenacious Defense states on the skill description. The shield should be recharged to 40% capacity after depletion instantly. INSTANTLY, not after proccing the rest of the damage to her health bar. If the skill doesn’t do anything to prevent you from taking damage the way it’s described in the skill description, it is therefor broken.
If the damage is so insanely high that it chews through that 40% recharged shield and deals damage to her health anyways? I might buy that. If it tears through it and then one-shots her into FFYL anyways, regardless of her being at max health or not? See my starting paragraph.