If an entity (in this case, the player) is standing on a magma block, they now take damage even if their hitbox is at the edge of the block due to the fix of MC-1133, which is expected.
However, there's still some strange behavior where even if you stand at the very edge of the magma block and there is any block below you other than air, you don't take damage.

Expected result
Logically, if the entity hitbox is (barely) touching the magma block, it would deal damage to that entity, regardless of if there is a block or air below the entity.

Observed result
Damage can be avoided if the hitbox touches the magma block, if there's any other block below the entity other than air.

How to reproduce

1. Place a magma block and stand on the very edge.
2. Notice how you take damage.
3. Now place a block directly below you without moving.
4. Notice how you no longer take damage.