I know this is a duplicate but all other posts are outdated.

Steps of the example :

1) Command block in the overworld (ticking area)

2) Put "/execute @a ~ ~ ~ detect ~ ~-1 ~ grass 0 say Grass" to check if a player is stepping on grass.

3) Put on repeat mode

4) Find a position where you step in grass in the overworld

5) Go to that position in the nether

Result :

The command block finds grass underneath the player and says Grass even in the nether where there isn't any.

Expected result  :

The command block shouldn't find Grass and not displaying grass.