Can affirm, happens to me all the time.
EDIT: Updating SimplyJetpacks to 0.3.0.0 fixes this problem, as there is a new config option to disable mobs' using of jetpacks.
Those are valid levels for where the farmland can be placed, not the bricks: "The table above is a side representation of both valid and invalid setups, regarding where you can place your farmland."