At a specific angle the sweeping attack is not working on cave spiders and only one mob takes damage. If the player is standing a slap higher it is working again.

The Spider are in a 1 * 1.5 * 1 hole. The Slab is on the ceiling. On one side there is a stair at the bottom (to get the spiders in) and a trapdoor ontop (so that the spiders can't get out).

The player is standing on the third block from the spiders, on the same level as the stair. The spiders have to climb up and be in the air, beneath the top slap.

If attacking from this position, there is no sweeping edge damage. Neither with a sweeping edge enchantment or not.

If the the player stands a slap higher, the sweeping damage is working again.

edit: changed "slap" to "slab"