The bug

In the structure minecraft:village/savanna/houses/savanna_large_farm_2, grass blocks are randomly present beneath the farm. This occurs seemingly randomly.

In the screenshot below, every brown mushroom block represents dirt, and every green wool represents a grass block that is out of place.

How to reproduce

1. Give yourself a structure block using the command /give @p structure_block
2. Set the random tick speed to 0 to ensure that the blocks do not change using the command /gamerule randomTickSpeed 0
3. Place the structure block, and set it to Load mode
4. Paste the following into the structure block's "Structure Name" field:

   minecraft:village/savanna/houses/savanna_large_farm_2

1. Prepare the position and then load the structure into your world
2. Use the F3 debug menu to see what type of block you are looking at