Did you rule out any stray electricity leaking into tank , i had that happen to me once when i was breeding angels. Somehow it affected only the fry , parents were ok. Culprit was a canister filter , after a few months of losing fry i swapped out to air driven filter and everything went back to normal.
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This happens in a lot of spawns when I was breeding angels. It always went back to a high bacteria load in the tank and that always went back to creeping nitrate levels. So, I'd say it's a water quality issue, though even in excellent quality water you can get higher bacterial loads and in that case a UV sterilizer works wonders. The other possibilities are parasites probably protozoans that are not serious enough to kill all of them or the parents but enough to kill the weakest of the spawn or, do a close-up inspection for hydra.
with my breeding of calicos i also noticed the same dorsal mutation. where either the very beginning of it was missing or it had 1 dorsal spike a gap and then the rest of the dorsal
It just takes so long for the fry to grow to know if there showing decent markings that ud want to breed that individual. I'm currently breeding a recessive carrier to a calico. And also 2 calicos w light striping n random markings 2gether. Just time consuming getting a surviving fry worth breeding
