The Thorn vines in the Rose Thorn caverns can be stunned and when stunned have a chance of dropping their unlock item.Which vines do I fight, aren't the ground vines invincible?
The excavator vines are indeed invincible and I've seen their item drop once or twice from the "hands" of big tree insect, random enemies and once or twice in random chests in the post endgame lily cave. Also pretty sure there's a regular chest somewhere that can drop them as well, but hard to recall.
The jellyfish items drop from the underwater jellyfish of course, but there is also 1 chest somewhere in the water of lower fairy land that dropped it for me (don't know if it's guaranteed or random from that chest).
You could try changing up the difficulty since it seems to effect primary enemy drops in a good way, but reduces the chance of other items (health items for instance, so potentially secondary item drops as well).
Sorry that I can't be of more help, as without info from the dev, or the source code I can't know the exact details of how the weird rng in this game works.