Mobs can attack diagonally through blocks. While the player can not see the mob, the mob will still attack and damage the player. This was tested on a superflat world in creative with cheats. The mobs that have been tested and that can attack through the blocks are: Zombies, Enderman, Slime, Wolf, wither Skeleton, Vindicator, and arrows if the player is close to the corner. Ghasts, Creepers, Silverfish, Shulker, Polar Bears, and Spiders can not attack diagonally. The mobs will also path find in order to attack diagonally.

I was expecting that the mobs would not damage the player through blocks.

Instead, the mobs will continue to attack through the block with the player being defenseless as swords can not attack through the block.

Steps:
1. Setup a 1 x 1 area blocked on the 4 cardinal directions with 2 high blocks.
2. Spawn a mob that can attack through the blocks (listed above)
3. In survival mode, the mob will being to attack the player