>Switching Projectiles Origins to Vanilla. Everything is fine with the Revenant: it shoots one missile.
Technically it shoots two missiles in exactly the same spot, it's a bit weird. This setting is purely for debugging and making compatibility work on maps that absolutely need the projectiles to appear at the original origin. In... 99.9% of scenarios, you should leave it off. The height check that makes the revenant fire the missiles from a bit lower if they would end up inside a ceiling works well enough.
>But the Arachnotron has not changed. I stayed with the same Triple Shots. Is this a bug?
No, it's the same as the revenant. Normally in the mod it fires from 18, 6, which is to say 18 units off the ground and 6 offset to the right. With the setting enabled, it fires from 32, 0 like it would in vanilla no matter what. Pic related, some code from the arachnotron.
This setting is only relevant where the difference between the "new" firing origin and the vanilla one would cause unintentional hits that wouldn't be possible in a map otherwise. For example, the logic for the arachnotron tries to see if shooting from 18, 6 will hit, and if it doesn't, it checks if 32, 0 will hit. If 32, 0 will hit but 18, 6 won't, it adjusts the offset to 32, 0 so it doesn't miss a shot that it would have landed in vanilla. However, the downside to this logic is that if a shot from 18, 6 will hit, but a shot from 32, 0 won't hit, you will get hit by a shot that might not have hit in vanilla.
After extensive testing I find that this usually makes zero difference except in cases where revenants can fire over a chest-high wall that they normally shouldn't. For maps with very tight balance, the setting exists to aid players in avoiding an unintentional difficulty spike from the result of a mostly cosmetic change.