Hello! I would greatly appreciate your(plural) help!

So my Cypress Vine seedlings (plantlings? since they have 2 true leaves) are dying! I started a bunch of seeds indoors to transplant outside. They grew with gusto, but the leaves turned light brown (starts spotty, then spreads), soften, wilt, then die. The root is gone too :( I've been starting seeds like this for many years, but this is the first year that almost all of my plants died!

I'm partially suspecting the weather - temperatures have been very fluctuating this spring in Central New Jersey, ranging from 80*- 40* all within a span of a week. I bring the plants indoors at night.

Any help / advice / tips would be greatly appreciated!