Steps to repeat
Create a level 7 Ranger with Hunter archetype and Multiattack defense.
Equip it with medium armor and shield to have high enough AC((let's say 18 AC with scale armor and shield) to easily notice whether the +4 works or not.
Pit against an enemy with at least 3 attacks, is possible 4.
Do unarmed attacks, let enemy attack.
Expected behaviour
When enemy hits on first attack, Multiattack defense activates and boosts AC by 4 (ex 18 AC -> 22 AC).
Witnessed behaviour
Enemy hit floor AC 18 on first attack, HITS ON SECOND ATTACK WITH SAME RESULT, (18), should have missed (18 < 22).
Additional notes
Noticed this when fighting Razan which was the first enemy I fought just after level 7, that was strong enough for me paying attention to hits & misses especially with Multiattack Defense. xd
I'm pretty sure I didn't mistake the attack chain with "Legendary action followed by regular turn" (in which case there is no bug since technically legendary action is on a different turn), although that may still be possible. I made a screenshot anyways, I'll upload it when I can (not on my "gaming machine" right now) and I'll double check then that I'm not reporting a false positive. :)