Arrogant self answer, because I have done conclusive Zombie science!
Whether Fighters drop down ledges or wooden platforms all boils down to the elevation of the player they're targeting. How near they are in the big picture doesn't seem to play any part in their decision (i.e., X position is ignored).
Three general rules:
- If the player is above, Fighters will jump off all ledges they encounter and will not drop down wooden platforms.
- If the player is below, Fighters will drop down all ledges without jumping and will drop down all wooden platforms they walk over.
Fighters on the same elevation behave similarly to when the player is above; however, against my expectations, if Fighters drop from a higher elevation onto a same-elevation wooden platform, they will pass through. Also, when faced with a decision between dropping through a platform and a jump, they will jump.
So, the reason the above pit let goblins coming from the left through: I was simply on the same elevation as they were, so they jumped it. Coming from the right, they dropped down onto the same-elevation platform and went through.
Armed with this knowledge, I present you a minimalist, player-friendly Fighter AI trap (sans lava and a way of actually collecting loot):
When standing in the middle during a Goblin Invasion, nothing except the Casters can touch you.
Of course, this design can be elevated to allow players some leeway for jumping and getting knocked around by enemy hits. Also, easier loot collection:
More annoying to navigate, though. 4 or 5 blocks high would be a good compromise.
(Disclaimer: Several dozen adorable Goblins, a few Zombies and a Bunny were harmed during the research for this answer.)