When you are executing based of an entity and detecting if there is a grass block under it, it detects the grass block under the player, rather the entity.

Steps to reproduce:
1. Open Minecraft 1.12-pre2
2. Make a new creative world
3. Do /summon creeper
4. Put a grass block under the creeper
5. Stay on a stone block
6. Do /execute @e[type=creeper] ~ ~ ~ detect ~ ~-1 ~ grass 0 say This should work
7. It says fail to detect
Another way
4. Put a stone block under the creeper
5. Stay on a grass block
6. Do /execute @e[type=creeper] ~ ~ ~ detect ~ ~-1 ~ grass 0 say This shouldn't work
7. It works, even though the creeper is on a stone block