Cobwebs can still generate unsupported near cave spider spawners

The bug

Although the changelog for 21w06a states that cobwebs will no longer generate unsupported (not touching any blocks), this issue is still occasionally present.

How to reproduce

1. Create a world with the seed 7624679963404965487

2. Run the command:

   /execute in minecraft:overworld run tp @s -1852.58 -42.88 -1094.12 821.19 45.41

   ❌ The cobweb in front of you is completely unsupported