...said the Wolf to the Three Little Pigs.
Stipulated: undue suspicion is as much a failure mode as insufficient suspicion. Both hazards might be implicated in any particular situation. That's why it's a mistake to make hard and fast rules. A sense of proportion is valuable.
AGF is a rule in the sense that if you don't know someone's bad faith, you must act as thought they have good faith as a rule for maintaining healthy discourse. It does not mean you assume good faith to the point of allowing yourself to be vulnerable or agree to anything based on that good-faith assumption.
In short: Assume Good Faith in how you communicate, but verify good faith with certainty if your decisions rely on it.