I was playing a Random Battle and my Honchkrow used Hidden Power Grass versus Colossoil (Turn 3). The following turn, I used it again due to being encored but on Turn 3 the move was super effective (like it should be) but on Turn 4 the move was not very effective. On turn 5 my opponent switched in his Piplup and my Honchkrow used Hidden Power "Grass" again, however it was neutral versus the Piplup.
I am still somewhat new to Pokemon, so perhaps there is something I am not taking into account and maybe this isn't a bug.
Here is the replay: http://play.pokemonshowdown.com/batt...dombattle42286