I'm watching a speedrun of Pokemon Yellow, and an Oddish used Absorb against a Nidoking. The game indicated that this attack is "not very effective", even though Ground's weakness should cancel out Poison's resistance.
It's been so many years since I've played Gen 1, but I have a vivid memory of my Venasaur's Razor Leaf also being NVE against Lance's Gyarados, even though a Water/Flying type should be neutral against it. I'm not sure how watch it said for Aerodactyl.
What are the known bugs in the way Gen 1 handled overlapping weaknesses and resistances in a dual-type Pokemon? Why does it show "not very effective" for these Pokemon when they should be neutral?