Steps to reproduce (a video is attached):

1. Place a building block and a cobweb above it.
2. Drop spiders or cave spiders from different heights into the cobweb.
3. They only receive damage when they fall from heights of 4 to 8 blocks above the building block.

Adding more cobwebs above the first cobweb does not change anything.

Either spiders should not take fall damage in cobwebs at all (which would be logical), or always take fall damage in cobwebs if they fall from a height greater than some value.