Reproduce the bug:

collect a 3 grass bundle (3 grass in 1 bundle)
move the bundle (3 grass) to a cart slot
collect 3 more grass
move the 3 grass to the same slot in the cart -

as a result the cart will show a 4 grass bundle - the back will be empty (2 grass are lost).

I believe the same will happen in a building (not tested)
I believe the same will happen with any other bundle (branches, twigs, metal, leather, plastic, cloth) ... if someone wants to try that - please add it to the bug.

I confirmed this bug with grass and carts (its consistent).