Despite not being a full block, chains support cobwebs. This can lead to a cobweb next to only chains (and no solid blocks).
How to reproduce
This seed and coordinates worked in 21w08a.
Seed: -8131653949012933360 Coordinates: /execute in minecraft:overworld run tp @s -876.69 -10.84 878.11 494.04 12.35
- is duplicated by
MC-218774 Cobwebs generate next to Chains