Mobs can still revert farmlands to dirt even if mobGriefing gamerule is set on false.

When a mob lands on a farmland and the gamerule mobGriefing is set on false, it can still revert it to dirt.

PD: The texture pack I was using was a customized version of Dokucraft The Saga Continues.
PD2: I think that affects to every gamemode.