I've only looted 5 Nigriventer Venom since launch, compared to 163 Gormar Venom. May be worth a look, unless I'm the exception somehow?
Mh, in fact the Nigriventers seem to be MUCH rarer than the Gornar. If that "spiders" drop linking is correct, it's a bit off-balanced, because spiders aren't killed much in ToV. Though, the Gornar's are more useful for tanks because of their additional aggro.
This is not a bug. If you want spider venom, you go kill spiders. The amount on non-spiders *IS* incidental and intentionally rare.