Two jellyfish issues:
First, when attacked with a fire dart, poison dart, or the obsidian speargun, the jellyfish will do 5 points of electrical damage, as though you were attacking with a melee weapon. The snakeskin armor will protect from this damage. (None of the other blowdarts or spearguns cause this issue.)
Second, when a dead jellyfish is on the ground and you pick it up, you get 5 points of electrical damage even if you're wearing protective armor, such as the snakeskin armor. I'm not 100% that's a bug, of course, but it seems weird to me that a dead jellyfish can get through armor that a live jellyfish can't.
Both these things were tested with all mods turned off. I did, however, use the console to spawn in all the weapons except the original fire dart.
Fire dart issue: 1. Make sure you're not wearing anything with electrical resistance. 2. Attack a live jellyfish with a fire dart, poison dart, or obsidian speargun. Lack of electrical protection issue: 1. Make sure you ARE wearing something with electrical resistance 2. Pick a dead jellyfish up off the ground.
A developer has marked this issue as fixed. This means that the issue has been addressed in the current development build and will likely be in the next update.