I disagree. Giving away your location is different from remaining unseen. You should be able to attack, and hide in the same spot again, as long as you still meet the vision requirements that allowed you to hide in the first place. Creatures will know you are there, but they will have disadvantage on attacks because they can't see you.
The game becomes WAY too complex and combat slows down if the DM allows creatures to fire arrows from light obscured areas while remaining unseen, but not hidden. The Perception checks to spot those foes are at disadvantage. Rolling disadvantaged perception rolls every round to spot creatures in lightly obscured areas is a pain.
Only a heavily obscured area states that the creatures are unseen (i.e. vision is blocked completely).
The concept that someone is standing in a fog and remaining unseen seems a bit off. I have no problem with an NPC hiding in the corner of a completely black room and darkvision not initially seeing him. I have more of a problem with once he fires his weapon, your darkvision does not spot him. He's in plain sight, not hiding behind anything. Hidden to begin with, but not once he fires his weapon. He's in plain view, but he is still in light concealment (i.e. dim light).
If he can see you with his Darkvision in total darkness, then you can see him with your Darkvision in total darkness.
It was the hidden state that allowed him to be unseen initially. That hidden state no longer exists once he attacks.
The problem comes in when one says that someone is hidden when their location is known automatically. The foe needs to hide again once he gives away his location in order to be hidden again.
If you move into a lightly or heavily obscured area, the creature has to make Perception vs. Stealth checks to see you.
No.
If you move into a heavily obscured area, you are automatically unseen and you cannot be seen with a perception check.
Also:
1) Lightly obscured areas do not block sight. Walking into moderate bushes does not make one unseen in order to require a perception.
2) Hiding requires an action.
3) Stealth checks are required when hiding, not when in lightly obscured areas and not hiding (i.e. position given away).
Seen until unseen.
Perception checks are only required in lightly obscured areas if the creature is hidden.
One of the main factors in determining whether you can find a hidden creature or object is how well you can see in an area, which might be lightly or heavily obscured, as explained in chapter 8.
The quote here states that the creature has to be hidden.
Someone standing in fog is not unseen. They are only hidden if they take the hide action. They might not be perceived initially (passive perception), but once they give away their position via attacking, they are now seen.
If they hide again, then they become unseen again and then it is Disadvantaged Perception vs. Stealth to find them. You might know their position, but they are unseen until you spot them or they attack again.
If creatures are in the bushes where they cannot be seen, then that is heavily obscured, not lightly obscured.
Lightly obscured means a few bushes in the way, fog, or dim light. It does not mean unseen. It only means unseen if hidden. Once attacking, a foe is no longer hidden (unless he is invisible or in heavily obscured, i.e. unseen).
I get your interpretation, but it is way too strong, a bit nonsensical (What do you mean that I cannot see the guy attacking me there in the fog? He can easily see me through the same fog that I cannot see him through. WT?), and it slows down combat.