When using execute on an entity, the executed command will be executed on the player (or command block) instead of the entity.

How to test
1. Open a world and find a sheep.
2. Run the command: /execute @e[type=sheep,c=1] ~ ~ ~ setblock ~ ~2 ~ stone
3. Stone will be placed 2 blocks above YOU instead of above the sheep
4. Chat will say [Sheep: Block placed] which means the sheep is running the command; its just relative to the player instead of the sheep itself

On previous versions of minecraft, the block would be placed relative to the sheep since the sheep is technically running the command via the execute. I can only assume this broke due to the addition of @s