Floating head (skull) caused by grass

If I place a head targeting a side of a grass block, then the head is placed as "floating" over the ground.
If I target the top side of a grass block then the behaviour is different, the head is correctly placed on the ground.

Same behaviour with the second block of a tallgrass.

I used a Wither Skeleton Skull for the screenshots, anyway it's the same with the other heads too.