WHY are my 1st instars dying?!

I've looked all over this reddit, google, you name it. This is bizarre. I've lost one caterpillar in a two batch previously, and now two in a current five batch. All the eggs hatch, they eat, they're doing fine... and then one of them will just go real still and give up.

I know what you're thinking: "maybe it's just getting ready to molt?"
But then I sit there and give it time, and I swear these babies are small, but they just get smaller (noticeably different compared to other egg siblings). No frass, no eating, no movement BESIDES the fact the head sometimes moves left to right and the butt lifts. Molting cats should attach their butts to their silk, no? I thought maybe the head movements were silk weaving too, but it just... it's almost like it struggles to move suddenly despite being just fine a day prior.

Notice that arching pose? The speck behind it wasn't there last night, I assumed maybe leftover frass but who knows at this point!!

AFAIK, no pesticides, no noticeable goo or black death, my cat isn't treated with any medicines, I put their mesh cage outside to get sun and shade, I spritz with rain water to keep the leaves moist.

Do they just sometimes... die because of weak genes? I've never had this issue before and it's making me so, so sad and worried. Am I doing something wrong? Am I killing them? I cannot find answers. I'm worried!

ANY help appreciated :(((