cloning grass path works just fine
example: cloning the block one to the right /clone ~1 ~ ~ ~1 ~ ~ ~1 ~ ~1
unless the block above gets cloned as well, even if it is air.
the block then just gets turned into dirt
example: cloning the block and the block above one to the right /clone ~1 ~ ~ ~1 ~1 ~ ~1 ~ ~1

*attached two screenshots for the examples

did some futher testing,
all versions above 1.10 are affected, in 1.9 and 1.10 it works as intended