If you spawn slimes using a soul cage, then take the shard out and kill the spawned slimes they initially don't count towards the number of souls.

But any slimes produced by a large slime splitting do count towards the souls when they are killed.

I'm not sure whether this is a bug or something that is intentional but figured it would be good to report it here.

Seem to have the same thing with any sheep bred from two sheep out of a soul cage, the babies do have souls when they die.

FTB Omnia
SoulShardsRespawn-forge-1.15.2-1.2.0-15