All I can say it can be explained by "the sixth sense", but again, what are you going to do with it?
And I agree, what happens when the "trigger" is resolved last?
If you see no one? Dodge actions? Cast a spell?
Then the ambushers can still do nothing and you waste your action.
When do you roll initiative again if nothing happens?
Next round?
after 3 rounds?
After 1 minute?
More?
when do the "out of combat" condition arrive?
Initiative is only rolled when an attack or other action that starts the conflict is declared. The arrow is drawn back and loosed, the first words of the fireball spell are chanted, the sword clears the sheath and is swung etc.
Now, it is unlikely, but possible that the character of the player who declared the attack would have thought that they got the drop on their enemies, but despite rolling with advantage and them rolling with disadvantage, they still roll a lower initiative. In which case they probably gave the game away by shifting position to aim, creating a noise, or some other stimulus. The player that declared the attack could technically choose to perform another action instead - aborting the shot when they realise that they have revealed the threat for example.
It is also possible to posit a scenario where there is
absolutely definitely no way of the victims being able to react to the attack: A long-range fireball into a group sitting in a both silence and darkness spells perhaps. In such a case the DM can adjudicate it the same way as any other check where there is no chance of success, and not allow a roll until the situation has changed and success is possible.
I like the new initiative system. The old one allowed parties to set up situations where they could alpha strike to such a level that the encounter was practically a cakewalk. In the new one, while the odds are in their favour, they still have a
chance to fail. Even if they do, after setting up an ambush, the odds are very much in their favour, since presumably they will still have cover/hiding etc.
All forms of initiative are going to have some issues given D&D's turn-based combat system. The important thing to bear in mind I think is that there are very few instantaneous actions. Ranged and melee attacks have travel time, spells take time to cast etc. In both media and real life there are examples of someone starting to do something but another person reacting fast enough to forestall them.