Question for those with a great deal of experience fostering kittens.?

Thank you for reading this. I'm a vet tech at a cat-only hospital and we were brought a litter of 6 kittens to foster about 2 weeks ago. We only have one left. Each kitten that has died has presented with the same signs and pattern leading to demise and as the vets we have aren't entirely sure what is up with them (not much fostering experience) I figured I'd ask those who have had many a tiny kitten in their care.

The litter was about 8 days old when brought in. 3 showed neurologic signs from the get-go (uncoordinated/uncontrolled circular motions of the head, often getting stuck in a dorso-flexion position.. maybe cerebellar hypoplasia from panleuk exposure?) and one of those was already extremely lethargic and had missed a couple meals. We attempted to tube-feed that kitten and it vomited a copious amount of pale white/green liquid that smell of feces and rot, and died later that afternoon. Over the next two days the other neurologic kittens also refused meals, vomited, and died.

That left the three "normal" kittens who were all eating well and growing. I had two of these kittens in my care. Then last sunday the lone kitten refused a meal, went lethargic, vomited that foul stuff and died. He'd been otherwise normal before that. This naturally made me nervous.

Yesterday morning my male kitten repeated that pattern, despite eating vigorously the meal before and toddling about with his normal energy level. I am now very worried that my seemingly healthy, eating, urinating, defecating and lone remaining kitten is going to follow her siblings. Have any of you encountered this before and if so, was a diagnosis made?