One of the things I've noticed lately regarding poisons is that, from time to time, something will go wrong and break a stack of poisons.
When this happens, that stack of poisons will cease delivering the intended effect, and will no longer be combinable with other stacks of the same poison. For example:

Both of these are carrion crawler poison, but the item reference ID has gotten messed up on the bottom stack, and it will no longer combine with the top stack and will not provide the on-hit effect.
I've been unable to determine what causes this to happen, but I think a workaround may be possible.
Item containers work by deleting items placed in them, and creating a record of the items, then creating new items when they're removed from the container. This might allow "broken" stacks to be replaced with freshly-generated functional stacks, if we let poisons go in herbalism bags.