Anyone have any idea what is causing this? I’ve looked it up and found that at least for younger saplings. Some Browning can be normal in early spring, but none of the images had nearly this much Brown. There is barely any green left among the needles.
Your best bet is to transplant it correctly in the ground this spring and let the plant get established this summer. Plants in pots are exposed much to more extreme temperature changes than if they are in the ground.
You can also cover them with some straw if the weather looks like it’s going for a sudden deep freeze and the trees are not covered in snow.