Ok, this one's weird, so bear with me. Have reported it before but not on here so thought I'd make it official.
Summary
Basically, if something has an immunity normally and is dual type, the Inverse battle will only register the typing with the immunity.
Explanation
In inverse battles obviously types get switched around, but the way this works with dual types with immunities is bugged. There are no immunities based on type in Inverse, ao immunities become super effective, the same as moves that are normally not very effective. This means ground moves are super effective against flying, poison's super effective against steel etc. I'll use Talonflame (fire/flying) as an example: In a normal battle, if someone uses a ground move on it, the fact that ground is super effective against fire is irrelevant, because flying is immune i.e. the game ignores the fire typing completely. However, in inverse it appears to keep doing this. If you were to use a ground move on Talonflame in Inverse, it would be super effective against flying, but not very effective against fire, cancelling out and doing neutral damage. The game however, seems to register that Talonflame would normally be immune due to its flying type, so proceeds to ignore the fire typing, meaning ground moves currently do super effective damage against it.
This seems to be the same for everything. More examples are that Ferrothorn should take poison moves neutrally, but at the moment they are super effective against it; Tropius should take 4x damage from ground moves but only takes 2x atm; Swampert should take electric moves neutrally but right now takes them super effectively.
This happens on both single player and multiplayer. I hope I have explained it well enough, as it is quite a confusing bug to get your head around.