When using the fill command to replace grass to stone. Sometimes no blocks were filled, as the tag enderman_holdable, is the new grass block. So in order to replace the grass block you must type "/fill ~ ~ ~ ~ ~ ~ minecraft:stone replace #minecraft:enderman_holdable"
I found out it was the enderman_holdable tag causing this, due to clicking F3 and hovering over the grass block,and seeing in the right side that the block is two blocks somehow:
"minecraft:grass_block
snowy: false

minecraft:enderman_holdable"