It looks like a bug with a cache. With this mod mobs do not drop crystals if they are dissolved in liquid death.
At the first launch everything is fine there are crystals. But with a repeated start of the game (when the mod loaded the cache), the vis crystals do not fall out of the monsters. Only ordinary loot.

Pyromanc3r, do you have Thaumic Wands installed, or Moar TCon? (Possibly both) Having a similar issue when both of these mods are installed, I load the modpack, then reload it.

This bug is present even in the vanilla TC6 + Thaumic Speedup. No other mods. At first I also thought that this was some kind of compatibility conflict.
But the problem itself in Thaumic Speedup :\