Baby zombies despawning as soon as they're not on the ground?

Apologies if this isn't the best sub to ask but I figure someone here might know what's going on. I wanted to farm the new music disc so constructed a general hostile mob farm (very standard flushing tower design), intending to filter out the babies for manual killing. The problem is that as soon as the babies - specifically the babies - are pushed off the platform and start falling, they despawn immediately. No other mobs (regular zombies, witches, skeletons, creepers) do this. They just seem to exhibit the normal occasional random despawning. It's just the babies that immediately despawn the second they're not on solid ground.

I'm on a server running nothing more than standard performance + tweaking mods (carpet, sodium, etc.), and I'm anywhere from about 50-90 blocks away from the babies by my estimate (distance varies depending on the height of which specific platform they're falling from).

What could be causing this? Is there something with babies specifically that changes their despawn mechanics?