Infestation error.
Venomoth using Infestation (Bug) on Vileplume (Grass/Poison) displays as Not Very Effective. Bug is super effective on Grass and not very effective on Poison. Why not normal damage? Also tested with Muk, same results.
Answers
It's not jut that set up as well, I seem to get "it's not very effective" when using an attack that is neutral against a Pokémon that has two typings, one which is weak to and the other resists the attack is using. I.e. Flash cannon against lapras, thunder shock against dragonite, karate chop against pidgeot, etc
0.714 x 1.4 = .9996
.9996 is less than 1 (neutral). So the message (not very effective) is probably designed to show up if the damage is under 1 (neutral), even if it is supposed to be neutral.
0.9996 is so close to 1 that it doesn't matter about the message, hp is being lost. Math is math.
There, the answer is above you... and now, below you.