Step to reproduce :
_Leave two spaces free in your creative hotbar
_Give yourself Dirt from creative inventory
_Give yourself Grassless Dirt (/give @p minecraft:dirt 1 1)
_Make sure you can distinguish which is which
_Place one bloc of each somewhere
_Use pickblock on both of them
_See how pickblock selects the Dirt block from your inventory in both case

(By the way, It doesn't help that both blocks are labelled as "Dirt". )