Immunity multipliers and how they stack

Edit: Question solved. It appears that Gengar's page is outdated and the damage Gengar receives from a poision type is:
IMMUNE * NVE = .51 * .71 = .36x.
This means that Mud slap rhydon will now be superior to rock smash against Moltres/Zapdos/Ho-Oh, because it gets STAB :)) that is if you were thinking about using it against these birds initially

S/o to the GP crew for putting in the work. Much appreciated! :)

This question is regarding the damage multipliers on Pokemon pages, specifically, those with two types-one which is immune to a certain type and another type which either resists or is weak against said type. For example, Charizard/Moltres/Zapdos/Aerodactyl & Gengar.

A while back, I believe* the aforementioned pokemon displayed a .51x multiplier (for immunity) for ground (and fighting for Gengar).

However, upon looking at these pages now, it appears the aforementioned pokemon now take .71x damage as flying + fire/electric/rock hybrids.

I was under the impression that the immunity multiplier did NOT stack with a multiplier of the second typing (which in these cases is either weak to or resists). In other words, it appears the birds'/lizard's resistance to ground has become essentially a NVE multiplier, where I originally thought this was not the case.

However I am even more confused because Gengar, with this logic, should receive .36x from fighting (ghost is immune to fighting and poison resists fighting), but his/her/its page displays .51x instead.

This leads me to believe that either:
- gengar's page has not been updated
- A floor multiplier is in place where immunity doesn't stack with NVE
- the flying + electric/fire/rock pokemon pages have been incorrectly updated (I doubt this is true)

My question is.. how do immunity multipliers stack when a pokemon has a second type that is either weak to or resists a type that its primary type is immune to?

Note: NVE = not very effective